Hello @pokko,
I hope you’re doing great today!
You can enable debugging mode by editing wp-config.php file:
https://developer.www.ads-software.com/advanced-administration/debug/debug-wordpress/
Then replicate the same error when saving the form, and check, if there’s a new “debug.log” file in /wp-content/ directory. It may contain some helpful details (a particular PHP error, references to any conflicting plugins/themes, etc.).
Another method of troubleshooting is to run a conflict test, by disabling plugins and switching the theme, as explained here. Please note, that it makes sense to run it on a separate staging site, so that it doesn’t affect the live site appearance.
I hope this helps. Let us know if you need further assistance.
Best Regards,
Dmytro