As of recently, it seems, when we try to Create an issue, the warning
"Some issue types are unavailable due to incompatible field configuration and/or workflow associations."
is displayed.

From reading as much as I can here and elsewhere, it seems like this is a fairly common issue with Jira, but on the other hand I don't believe I changed anything, especially recently, that would cause this. To be honest I don't feel I know enough about customizing Jira at this point that I should be able to cause some configuration issue.
Also recently — and I don't know if it's related, but it seems like it started happening at the same time — we've become unable to create an issue using the Jira mobile app (on Android) at all. The error in that case is:
"Can't create the issue in this project. Try a different project."

Except there is no different project. And this, also, is something that used to work.
Does either of these things — or particular the combination of the two — suggest somewhere I should start looking?
Some additional information:
I can create, it seems, a "Task" or an "Epic" using Jira Mobile for Android, but anything else — "Bug", "Improvement", or "New Feature" — gives me the "Can't create the issue..." error.