I'm dealing with the same issue since days. No matter what changes I make and what type of user I edit, it won't save the changes and reports the Error "500" (Internal Error). It's acutally a major problem and a big deal. I won't use FengOffice under those circumstances, because it would mean that I can't make any changes of personal information or permissions after a user is created.
Here is a solution I found, but it doesn't work for me. Maybe some of you guys try it out and could report if it worked.
---Can't insert links. So just look for "Topic: Can't change user permissions" in this forum. It will lead you to a github-link and tells you to make changes in permission.php - file.
I use FengOffice 3.1.2.2 and run it on a Synology Diskstation. It works great, I had no problems with installing and it works fine in general. But this is an important issue and I can't believe it's not fixed yet. The error "500" even occurs in the fengoffice demo version of softaculous.
Please try the solution above and report your results. I have the feeling, that I do something wrong and I may not understood the solution fully