Having a similar issue with the certificate mod in M2.3 - seems like some new instances of the activity, won't generate a certificate. I can see that news rows are written to the certificate_issues table, but no certificate. It just times out. I get this error with debug on:
Fatal error: Maximum execution time of 120 seconds exceeded in C:\inetpub\wwwroot\ecbhintranet\lib\dml\mysqli_native_moodle_database.php on line 902
I'm really stuck. I have another site, same code base on the same server, which is running fine with the certificate module. So I guess I have some DB problem? Thanks for any follow up on your fixes...