[RAD server modpack drops an error while I try to join]

This is what console threw out after crashing

> 2020-07-01 18:34:38,739 Server Shutdown Thread ERROR An exception occurred processing Appender Console java.lang.IllegalStateException
    at org.jline.reader.impl.LineReaderImpl.callWidget(LineReaderImpl.java:651)
    at net.minecraftforge.server.terminalconsole.TerminalConsoleAppender.append(TerminalConsoleAppender.java:308)
    at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:156)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:129)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:120)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:84)
    at org.apache.logging.log4j.core.appender.rewrite.RewriteAppender.append(RewriteAppender.java:83)
    at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:156)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:129)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:120)
    at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:84)
    at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:448)
    at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:433)
    at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:417)
    at org.apache.logging.log4j.core.config.LoggerConfig.logParent(LoggerConfig.java:439)
    at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:434)
    at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:417)
    at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:403)
    at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:63)
    at org.apache.logging.log4j.core.Logger.logMessage(Logger.java:146)
    at org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2091)
    at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:1988)
    at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1960)
    at org.apache.logging.log4j.spi.AbstractLogger.info(AbstractLogger.java:1297)
    at net.minecraft.server.MinecraftServer.func_71260_j(MinecraftServer.java:420)
    at net.minecraft.server.MinecraftServer$4.run(MinecraftServer.java:1579)

This is what crash report said about it

Time: 2020-07-01 18:34:38 CEST
Description: Exception ticking world

java.util.ConcurrentModificationException
    at java.util.LinkedList$ListItr.checkForComodification(Unknown Source)
    at java.util.LinkedList$ListItr.remove(Unknown Source)
    at net.minecraft.server.management.PlayerChunkMap.tick(SourceFile:148)
    at net.minecraft.world.WorldServer.tick(WorldServer.java:227)
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:756)
    at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:397)
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:668)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526)
    at java.lang.Thread.run(Unknown Source)

Version of Server Forge is forge-1.12.2-14.23.5.2838-universal and server is running on 1.12.2. Please help, I was trying 3 hours to figure out what's going on, if it's because of some mod but nothing works.

[Will turning off background apps give Minecraft more fps?]

yes, it will, but it also depends on what graphic details your Minecraft is running and how much RAM you have.

[Do I need to ask mojang or make a book about it?]

no, minecraft have actually done a few books :).

[mcp libraries missing - 1.10]

So I recently have tried to try something new and change minecraft 1.10 version code. Of course, I have done decompiling. There was only one error, not counting scala - 
1 out of 1 hunk FAILED -- saving rejects to file 'net\minecraft\util\math\Cartesian.java#'

ok I thought, it won't do anything serious. Then there came second error.
 

'"C:\Program Files\Java\jdk-14.0.1\bin\javac" -Xlint:-options -deprecation -g -source 1.6 -target 1.6 -classpath "jars\versions\1.10\1.10.jar;lib;lib\*;jars\libraries\com\google\code\findbugs\jsr305\3.0.1\jsr305-3.0.1.jar;jars\libraries\net\java\jinput\jinput\2.0.5\jinput-2.0.5.jar;jars\libraries\org\lwjgl\lwjgl\lwjgl-platform\2.9.4-nightly-20150209\lwjgl-platform-2.9.4-nightly-20150209-natives-windows.jar;jars\libraries\com\ibm\icu\icu4j-core-mojang\51.2\icu4j-core-mojang-51.2.jar;jars\libraries\org\apache\httpcomponents\httpcore\4.3.2\httpcore-4.3.2.jar;jars\libraries\org\apache\logging\log4j\log4j-api\2.0-beta9\log4j-api-2.0-beta9.jar;jars\libraries\org\apache\commons\commons-lang3\3.3.2\commons-lang3-3.3.2.jar;jars\libraries\net\java\dev\jna\jna\3.4.0\jna-3.4.0.jar;jars\libraries\com\paulscode\libraryjavasound\20101123\libraryjavasound-20101123.jar;jars\libraries\net\sf\jopt-simple\jopt-simple\4.6\jopt-simple-4.6.jar;jars\libraries\com\google\guava\guava\17.0\guava-17.0.jar;jars\libraries\oshi-project\oshi-core\1.1\oshi-core-1.1.jar;jars\libraries\org\apache\httpcomponents\httpclient\4.3.3\httpclient-4.3.3.jar;jars\libraries\org\apache\commons\commons-compress\1.8.1\commons-compress-1.8.1.jar;jars\libraries\it\unimi\dsi\fastutil\7.0.12_mojang\fastutil-7.0.12_mojang.jar;jars\libraries\net\java\dev\jna\platform\3.4.0\platform-3.4.0.jar;jars\libraries\com\paulscode\codecjorbis\20101023\codecjorbis-20101023.jar;jars\libraries\com\paulscode\soundsystem\20120107\soundsystem-20120107.jar;jars\libraries\com\paulscode\librarylwjglopenal\20100824\librarylwjglopenal-20100824.jar;jars\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.4-nightly-20150209\lwjgl_util-2.9.4-nightly-20150209.jar;jars\libraries\commons-codec\commons-codec\1.9\commons-codec-1.9.jar;jars\libraries\net\java\jutils\jutils\1.0.0\jutils-1.0.0.jar;jars\libraries\commons-logging\commons-logging\1.1.3\commons-logging-1.1.3.jar;jars\libraries\org\lwjgl\lwjgl\lwjgl\2.9.4-nightly-20150209\lwjgl-2.9.4-nightly-20150209.jar;jars\libraries\commons-io\commons-io\2.4\commons-io-2.4.jar;jars\libraries\com\mojang\realms\1.9.1\realms-1.9.1.jar;jars\libraries\com\mojang\authlib\1.5.22\authlib-1.5.22.jar;jars\libraries\com\google\code\gson\gson\2.2.4\gson-2.2.4.jar;jars\libraries\com\mojang\netty\1.6\netty-1.6.jar;jars\libraries\com\paulscode\codecwav\20101023\codecwav-20101023.jar;jars\libraries\net\java\jinput\jinput-platform\2.0.5\jinput-platform-2.0.5-natives-windows.jar;jars\libraries\org\apache\logging\log4j\log4j-core\2.0-beta9\log4j-core-2.0-beta9.jar;jars\libraries\io\netty\netty-all\4.0.23.Final\netty-all-4.0.23.Final.jar" -sourcepath src\minecraft -d bin\minecraft @temp\java_src_client.txt' failed : 2

== ERRORS FOUND in JAVA CODE ==

error: Source option 6 is no longer supported. Use 7 or later.
error: Target option 6 is no longer supported. Use 7 or later.
==================

Initial client recompile failed, correct source then run updatemd5

 

There I got scared. I've downloaded Eclipse. I've got into the files, I had everything. But. I couldn't run the minecraft. There were two errors. One was about the server:
Project 'Server' is missing required library: 'jars/minecraft_server1.10.jar'

And the second one:

Project 'Client' is missing required library: 'jars/libraries/com/mojang/realms/1.9.0/realms-1.9.0.jar'

I've added the file realms.1.9.0 to the path the Eclipse gave me with client. I've put the server files. And then, when I tried to run the minecraft one more time, there came, I don't know from where, these errors (see png's attached)
Does anyone here know how to get these missing libraries or what the hell is that?