Infinite load when loading world in Forge 1.12

I tried loading a vanilla 1.16.5 world in Forge 1.12 with mods, but the world is stuck on the loading screen and the only way to close the game is by using the task manager. Here is the crash report that follows:

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 2/5/21 10:58 PM
Description: Exception in server tick loop

java.lang.IllegalArgumentException: ChunkNibbleArrays should be 2048 bytes not: 0
    at net.minecraft.world.chunk.NibbleArray.<init>(SourceFile:16)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75823_a(AnvilChunkLoader.java:447)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.checkedReadChunkFromNBT__Async(AnvilChunkLoader.java:139)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.loadChunk__Async(AnvilChunkLoader.java:104)
    at net.minecraftforge.common.chunkio.ChunkIOProvider.run(ChunkIOProvider.java:69)
    at net.minecraftforge.common.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:92)
    at net.minecraft.world.gen.ChunkProviderServer.loadChunk(ChunkProviderServer.java:118)
    at net.minecraft.world.gen.ChunkProviderServer.func_186028_c(ChunkProviderServer.java:89)
    at net.minecraft.world.gen.ChunkProviderServer.func_186025_d(ChunkProviderServer.java:135)
    at net.minecraft.server.MinecraftServer.func_71222_d(MinecraftServer.java:343)
    at net.minecraft.server.integrated.IntegratedServer.func_71247_a(IntegratedServer.java:142)
    at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:156)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:483)
    at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
    Minecraft Version: 1.12
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_51, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 1311821448 bytes (1251 MB) / 2147483648 bytes (2048 MB) up to 2147483648 bytes (2048 MB)
    JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
    IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
    FML: MCP 9.40 Powered by Forge 14.21.1.2443 6 mods loaded, 6 mods active
    States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
    UCHIJAAAAAAA    minecraft{1.12} [Minecraft] (minecraft.jar) 
    UCHIJAAAAAAA    mcp{9.19} [Minecraft Coder Pack] (minecraft.jar) 
    UCHIJAAAAAAA    FML{8.0.99.99} [Forge Mod Loader] (forge-1.12-14.21.1.2443.jar) 
    UCHIJAAAAAAA    forge{14.21.1.2443} [Minecraft Forge] (forge-1.12-14.21.1.2443.jar) 
    UCHIJAAAAAAA    mercurius_updater{1.0} [mercurius_updater] (MercuriusUpdater-1.12.jar) 
    UCHIJAAAAAAA    imsm{1.12} [Instant Massive Structures Mod] (1.12.InstantMassiveStructuresMod.jar) 
    Loaded coremods (and transformers): 
    GL info: ~~ERROR~~ RuntimeException: No OpenGL context found in the current thread.
    Profiler Position: N/A (disabled)
    Player Count: 0 / 8; []
    Type: Integrated Server (map_client.txt)
    Is Modded: Definitely; Client brand changed to 'fml,forge'

Can someone explain what is happening here and if there is a way to fix it? Thanks in advance.