I just inherited care and feeding of a website that uses JEM 2.2.3 on Joomla 3.9.1 with php 7.0.33 . I've never used JEM before and didn't set this website up.
JEM is configured so that events can only be created/modified on the backend by Joomla users. When Joomla users try to add new events or modify existing events, they simply get a blank page. I'm a superuser and I get the same thing: just a blank page.
For example, I click on "Add event," and it opens this URL : $WEBROOT/administrator/index.php?option=com_jem&view=event&layout=edit , but all this is is a blank page.
I turned on Debug System logging but couldn't find any error messages. Contents of jcontroller.log.php:
When I try to edit event ID 102, this is all that is written to jcontroller.log.php :
#Fields: datetime priority clientip category message
2019-11-02T19:10:31+00:00 INFO 188.8.131.52 controller Holding edit ID com_jem.edit.event.102 Array (  => 102 )
Something else extremely weird: when I do "view source" on the above blank page, the source is all JS and it seems to stop in the middle of a script: