Update: Its not the clickwheel that’s broken, its the lock-switch - regardless of its position, the iPod considers himself in the “locked”-state. Replacing the headphone-connector and the ribbon cable did not fix it - so still two defect ipods here.
Replacement went well, with 2 exceptions: 1st, I would recommend to keep the headphone jack cable in place, and rather unscrew the connector from the metal backside, as the connector got damaged (fragile piece vs. lot of force). 2nd, after replacment the clickwheel is not working any more. I suppose it got damaged by ripping out the old battery, that is glued to the backside of the wheel. I could “reproduce” it with a second ipod, which i ordered for the replacement parts. So now i have two ipods not reacting to the clickwheel, one with new battery. Well - take care of the clickwheel and be carfule when removing the old battery!