This is an error you might experience while trying to generate proxies for EP. This can be caused by having a table field setup with an enum that does not exist.
System enums, i.e. enums not in AOT / Data Dictionary / Enums, seems to be falling in the category if enums that "doesn't exists".
You can read a bit more about the issue here.
The error message it self doesn't reveal much information, so the fastest way to find the culprit seems to be to run a Best Practice Check for all your tables. The Best Practice Check will report the issue.