the last Nextcloudupdates sucks… This was the reason, why i switched from OwnCloud to Nextcloud… 
Anyway. Same Problem on Raspian (Debian) 10 with PHP 7.3 from the Repository.
This help:
after this, maybe you need also a reload of the Apache2 / Webserver like this one:
service apache2 reload
I’m not sure about it.
Thank You, Community!