If FVTT is trying to install a module, it will download the zip file into the modules directory, if it then fails to unzip it due to lack of space, the module might not be unzipped entirely, and without the module.json will not be recognized as a module directory, making it impossible to delete it through the interface.
The zip file that was downloaded would also in that case remain as it would not get cleaned up.
closing since we have the delete orphans tool