XF 2.1 "Oops! We ran into some problems." error when trying to post new thread


Well-known member
When trying to post a new thread on my forum, I get the following error message:

Screen Shot 2019-09-17 at 12.45.07 PM.png

This doesn't always happen, only randomly, without any rhyme or reason.

When I check the error console in my browser, I see this:

Screen Shot 2019-09-17 at 12.49.48 PM.png

Any idea what's going on?