DB: No such field! Error message
Try installing the CiviCRM Log Viewer extension (you can do that via the web interface) and then you'll be able to see more details about the specifics of the error, which are logged in a CiviCRM error log file that is otherwise not easy to access.
The 'Transaction integrity error' isn't super useful, it's evidence of a deeper issue - see:
https://civicrm.stackexchange.com/search?q=Transaction+integrity+error%3A+Expected+to+find+active+frame
That deeper issue would be more easily seen via the error log.
Sometimes, if you make changes to custom fields via the web interface, it will only half-complete, leaving a mismatch between the tables in mysql and what CiviCRM thinks is there. That might be evidence of a server setup issue, for example.
The easiest solution, if it works, is to delete the fields that make up the custom fieldset, then delete the fieldset itself and start again.