Try to deactivate the other plugins on your site. As the code of 3CX webinars hasn't been tested with the last 3 major versions of WordPress nor does it support latest PHP, it might cause problems. We had problems with both iThemes BackupBuddy and cminds tooltip glossary. After deactivating those plugins, 3CX webinars worked again. But those plugins are up2date, whereas 3CX webinars hasn't been developed further for 11 month. So the root cause might be 3CX webinars.