It could be many things but for me often this most usually is an error in my conditions, that doesn’t come up in the issue checker. For example referencing a deleted Thing. Or an error that the issue checker hasn’t logged yet because it is on a Bubble page or reusable that you haven’t opened in a long time.
Or looking at this - it looks it could be a 3rd party service - maybe it is sometimes unavailable? Which might show up in the Bubble logs?
Have a look for any generic workflow that is catching errors - maybe that is hiding the real cause of the error from you?
And this thread has some helpful tips on looking in the Developer console - Recurring error in an app developed on Bubble - #5 by tatyana.zablotski
Or it is an issue in Bubble … fun fun fun … and then it’s Bug Report time Bug Report | Bubble