It's a simple site I run for my american legion post. I only create events on the admin part of the site, not on the public available side. After the joomla and jem version update I haven't been able to edit or create events and just tried to create a new category and get the same issue. I can create/edit venues.
That's really strange. Because you are saying nothing about your system (template, additional plugins etc. I think you are trying on a clear JEM 2.2.3 on a clear Joomla! 3.9.2 with protostar. With that you shouldn't have problems. I don't have problems.
If you are not on clear Joomla! and JEM, then you should give more informations. Mostly there are some additional plugins or templates which causes such problems. Buth without any informations, we can't and won't help.
PHP 7.2.9 on different sites
JEM 2.3.0dev 2.0M and specials with Joomla! 3.9.3 stable on livesites
ja_simply, ja_purity_iii, multilingual DE EN IT FR
The site worked fine with everything installed until both the site and your software update.
Site template is: Kirigami v1.12 RocketTheme Joomla Template Club I use this exact template, jem on several other sites and they work fine together.
other than the joomla files, this is what is installed on the site: