To given an update here after debugging this. It turned out the server is just very slow to respond sometimes due to disk I/O being slow. There is no real solution besides increasing timeouts, which may cause all kinds of other side-effects.
The error reporting inside the minecraft admin webui could be improved to give more indication besides internal server error though, have to see when I find time for this though.
Forge and Fabric are incompatible, but you can use Fabric/Forge mod with Vanilla Datapack.
Optifine is a alternative graphic engine for Minecraft that most of the shaders are made for.
Optifine is not compatible with fabric, but can be load in to Vanilla Minecraft or Forge Minecraft.
Cloudron support just Vanilla Datapack, not any other mod loader (forge or fabric) but been Optifine Client-Only it will work.
But if u wont to add mod that you find on Curse/Twitch you need to w8 later September we will release some version of minecraft apps for cloudron that support Forge/Fabric mod.