What we mean by making a server side mod is making a mod thast uses server logic to manipulate the containers directly instead of making a mod that automates things client side, which would be classified as a cheat mod and would make us very sad
If you have MixinBootstrap installed try removing it, it's a mod made to provide mixin support when Forge can't or doesn't... but it's not smart enough to disable itself when Forge DOES provide mixin support... and they conflict
Do you have Optifine installed? If so, remove it from your mods folder
If not, please provide debug.log (more info on where it is and how to provide it can be found in my signature below)
Yeah, usually AFK kicking is a server administration thing and is done for a reason... auto-reconnect defeats the purpose and on some servers might be considered cheating
If your issue is [0x10008]Cocoa: Failed to find service port for display it's not something we can fix, we have to wait for it to make its way down the chain through LWJGL and Mojang to us and it's unlikely to be backported
So when you said "without mods"... you still had Optifine? Please note that Optifine is a mod, and quite an invasive one.
Please refer to Optifine's changelog for compatibility information, as it is on the closed-source project to be compatible with the open-source one, not the other way around.
After looking twice those timestamps are not the same, and it looks like onLeftClickBlock fires for both start AND continueDestroyBlock, try getting the blockstate's destroy progress in your handler to check