I had previously reported this problem as a bug in MDL-67358 and closed as a supposed duplicate (which it is not).

Installation of mod_groupformation is breaking JavaScript in Moodle 3.8, if the JS-Cache is activated (which it usually is).

The plugin seems a little bit outdated - this bug report is just to make sure, the error is documented, as we needed some time to find the plugin to blame. Especially when a Moodle installation is updated, this plugin may break a working installation without any hint, why and what is responsible!

Anyway the problem is documented in the Plugins Git-Repo and it seems to me that people are working on it.