I was researching this topic a bit deeper. All answers found on this SE suggested disabling Gutenberg with a plugin. This couldn’t be a valid “fix” in my oppinion.
After researching and browsing through the git issues of
WordPress/gutenberg I’ve found a pretty easy solution for this problem.
joshuafredrickson on the git suggested changing the args of the custom post type array from
'public' => false, to true.
I have checked that fix on multiple of my clients projects and it has worked every single time.
solved The editor has encountered an unexpected error. // TypeError: Cannot read property ‘prefix’ of null