Wifi-Lan Bridge on OM2P not working after upgrade

Hi openwrt forum!
I've got some weird problems with an OM2P device (https://openwrt.org/toh/openmesh/om2p).
First it had some old openwrt version running (from 2017 probably), running as transparent lan-wifi (ap mode) bridge (so just the bridge with wifi and eth interface).
It worked but due to a kernel bug (afaik) sometimes the lan port stopped working ("eth0 tx timeout") completely.
So I upgraded to 21.02.3 which went fine, but the bridge did not work as before.
The lan clients can reach the gateway itself via the wifi bridge, but nothing external anymore through the gateway. The device itself also has the same problem. It can reach the internet through eth0/1, but not through wifi (it can reach the gateway though).
I tried many configuration options already (such as accept local, promn. on the interfaces, firewall rules, compared to the old config I saved before upgrading, different bridge interfaces). Once I read that relayd should be used, but before it worked as normal bridge.

So I thought I should reverse to the AR71xx state / 19.07 version.
But that won't work. Not with command line (also w/o settings), not with browser.
The 21.02.3 version gets booted after flashing / power cycle.

Does anyone have a hint?