Your resource pack is likely too high resolution or contains files that Eaglercraft's JavaScript engine cannot parse. Try a lower resolution pack (16x or 32x).
Here is your step-by-step guide to adding mods, resource packs, and shaders to Eaglercraft. how to add mods on eaglercraft
: Open an Eaglercraft HTML client that has EaglerForge pre-installed or use the EaglerForge Injector to create one. Your resource pack is likely too high resolution
Be careful when downloading clients or plugins. Only use reputable sources like official GitHub repositories to avoid malware. how to add mods on eaglercraft