Hi, JBW! Thanks for your quick reply!
Permissions for cache folder were 600. After setting to 755 the error become
(1/1) IOException
Cannot rename "/tmp/removed-ids.phpCdMEsd" to "/home/ngnailsd/public_html/ngn-de-shop/var/cache/dev/ContainerM1pd5t9/removed-ids.php".
It seems that all the files and folders that are created there are set to 600.
Meantime I checked the error_log and I found this: "[29-Apr-2020 08:46:49 Europe/Berlin] PHP Fatal error: Uncaught --> Smarty: 0():Missing '$template' parameter <--
thrown in /home/ngnailsd/public_html/ngn-de-shop/vendor/smarty/smarty/libs/sysplugins/smarty_internal_templatebase.php on line 177"
This was before I started seeing error 500.