sorry for the late reply, it seems that I tried updating my mod to 1.15 last time and it was unsuccessful. All the references to the version say 1.15 (even though I know it was never updated to 1.15).
My mod doesn't have a custom config, a config handler or any other mods in the workspace. (I also haven't been back in my workspace in a while so all these comments are from what I can find/remember)
My mod has been launching fine, until just recently when Eclipse has been giving me this error:
I have tried launching using a "runClient.bat" with the code "/gradlew runClient" to no avail.
All help would be most appreciated, thanks.