Ah, the error message is telling us what is wrong, but unless you've seen it before, you probably can't work it out from there. The logs would tell you more, but even then, take you on quite a roundabout route to what the problem probably is.
I mean if you go chasing the logs, you'll probably find (after three or four hops) a database error, which still doesn't tell you the real problem, but at least gets you to look at the database. And I'm pretty sure you are using MySQL for the database!
If this is what I think it is:
It's not your users that are a problem, it's the content they are putting into Jira, combined with a configuration quirk.
I think that you'll find the content they are trying to put into a text field (summary, description, comments etc) contains an advanced emoji or some other "clever" encoding. And your database is not set up to handle it. So when they post the data, Jira hands it to the database, and the database goes "nope, not doing that", so Jira comes back with a "can't create issue" error.
I first ran into this with MySQL where someone was pasting in that "smiling poop" emoji. Take the emoji out, the create/edit worked fine.
So test that next!
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