The issue was much more straightforward than i thought. It seems sometimes thinking of too complex issues will hinder finding the easiest cause - the local forewall on the pi was blocking it / had no explcite allow.

To check i did: sudo ufw status verbose

There was only port 22

I added the new port as Allow Port 8081: sudo ufw allow 8081

And it works now! Thanks for all the tipps that pointed me in the right direction!

Thanks for the hints, this definitely helped, however it did not solve the issue.

What i did:

1. I changed via omv-firstaid the omv port from 80 to 8081.
2. I confirmed with ss -ltn that this change was successful and i see the listening port 80 vanished, while this now popped up:

State Recv-Q Send-Q Local Address:Port Peer Address:Port

LISTEN 0 511 0.0.0.0:8081 0.0.0.0:*

3. I tested locally via ssh from the pi the connection via curl http://mylocalip:8081/ and it works, i get the html back
4. I tested from my laptop (connected to my router via WiFi, where the raspberry is meshed into via the repeater in between) and i still get the timeout.
5. I tried tunneling again via ssh ssh -L 8081:localhost:8081 pi@raspberrypi.local and i did not get any errors this time. However when i open the local url in the browser i get a connection reset and my terminal shows me channel 3: open failed: administratively prohibited: open failed. However this just says that TcPForwarding is disabled, which is fine, so that tunneling issue should not be the main problem, i assume.