Hi there presis, welcome to the forums! Sorry you’re hitting some trouble with categories not saving when an event is submitted using the Community form, but let’s see if we can tackle it together. 🙂
I wasn’t able to replicate the issue on my end but that doesn’t mean it’s not an issue for you. The first things I’d like to try is testing for conflicts with your current theme and other installed plugins. Could you try temporarily activating the default Twenty Thirteen theme, to see if the issue happens there as well?
If disabling the theme fixes it, that’s good news! It means we have narrowed the issue down to a theme conflict.
If the issue is still there when using the default Twenty Thirteen theme, then we can try testing for conflicts with other installed plugins. Using the Twenty Thirteen, please try deactivating all of your plugins, except for the ones from Modern Tribe. Does that solve the problem? If so, reactivate the plugins one at a time until the issue pops back up, then tell me which one was causing the issue.
These steps are also outlined here if you want some more details on how to test for conflicts.
Give those steps a shot and let me know what you find.
Cheers!
Geoff