Thanks for your help! I changed one thing to try it out as I had the IP adress of my Jail on the same IP as the Truenas (VNET on; 192.168.10.88:8283). Last night I played with different IP settings and couldn’t get it back to work with VNET. So I disabled VNET in the jail and assigned a static IP adress (192.168.10.100) which worked fine until today.
Now I tried to set the IP to DHCP (192.168.10.91) and when I now use Nextcloud, everthing works perfect again. What could be the problem with my currently used IP?