On a fresh install of the mod, I'm getting this error:
[STDERR]: java.io.FileNotFoundException: ./config/enderitemod.json

Afterwards, no config file is created either.

Edit: Apparently the config folder isn't created automatically by fabric. You also have to check if this folder exists, and check for the config file itself.
On my test, the config folder did exist already.

Oh, yeah, I think it should be fixed in the next release. kyrptonaught made a pull request which should fix it.

Confirmed it's fixed. Thanks!