2,5 months later, I noticed three of the spokes were broken close to the hub. Went back to the shop and had the spokes replaced (free or charge). 1 month later I noticed 2 broken spokes and had those replacereplaced as well. They seemed less eager to keep fixing the wheel free of charge, and when I asked why the spokes kept breaking, the guy muttered something about the hub holes maybe had burrs due to wear.
Once again, thank you all for your input!
Anecdotal Update
In case anyone follows... Since the last repair at the other shop, the wheel kept being in good shape. Finally, my spokes stayed tensioned, wheels stiff and true - the long struggle was finally over. Alas, the joy didn't even last a month...
Going home from a company party, I returned to my bike I parked at the train station, only to find out that some punk kids had apparently tossed it to the ground, and jumped both wheels badly out of shape. Front wheel had to be replaced and back wheel was in desperate need for a trueing. sigh
Sorry for the melodrama, just thought I'd update you the faith of my bike ;-).