The error you're getting with SSH isn't a problem with the pi, your local user is not root which means you cannot bind to any port <=1024, try listening on local port 8080 instead with -L 8080:localhost:80

bind [::1]:80: Permission denied channel_setup_fwd_listener_tcpip: cannot listen to port: 80 Could not request local forwarding

Some service is still grabbing your port 80, so the new job cannot start to listen there.

However i can not connect to port 80 for OMV

But this service is not really listening.

Maybe your first installation job is not 100% finished, but still 'hanging around' somehow?

Just a thought - I do not really know OMV.

The ssh tunnel might not work because the linux os reserves the ports 0-1023 to OS. You need higher priviliges to reserve this ports for your applications. If you type sudo ss -tulpn you can see the applications that are bound to you ports. Do you see nginx behind local addresses 0.0.0.0:80 and [::]:80 ?

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.

I don't use OMV so take this with a grain of salt, but I would hazard a guess that the web server isn't listening on port 80.

Try ss -ltn for a list of ports on which the system is listening and ss -nut for a list of active connections. Double-checking firewall rules (commonly ufw) or filter rules (iptables) will be useful for diagnosing connection issues.

(edited swapping around ss option explanations)

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!