Indeed it is DNS rebind protection.
I use wildcard on my Fritzbox. I added *.mydomain.com
and my MyFritz address *.er467.myfritz.net
Important for everyone who makes changes on DNS rebind protection: You will need to completely RESTART your Fritzbox. Even if you add a line, or edit. One domain per line. It will not work without it ☝🏻
You need to export your DB and impirt it again. You can not simply upgrade. Switch back to v15 and make a dump.
Here is a little guide.
Pict-rs can be upgraded simply. Edit your docker-compose.yml and spin it up. After tge upgrade is complete, you can do the postgres upgrade