When I add a html5 banner to a campaign in Revive 4.1.1 and click 'save changes', I get an empty page in the admin. The Apache error log contains:
What may be causing this?
I'm using Revive 4.1.1 (upgraded from 4.0 and OpenX before that) with PHP 7.0 and MySQL 5.5 on Debian 9.2 Linux.