I tried it here too, with similar results, and found no logging when I started Gramps from terminal. The error is quite clear though, if you know what the tool does, and that is documented on the wiki, where it says that the add-on uses the Nominatim service, provided by openstreetmap.org, a.k.a. OSM.
The error suggests that the sever has a problem, and without extra logging, it’s not clear what’s causing that. It might be a simple problem on their side, but it might also be triggered by a command that Gramps sends. We’ve seen other services that changed, and this add-on is pretty old.
There is an easy remedy though, and that’s forgetting the whole thing, and some other place tools too, and use the Place Cleanup instead:
https://gramps-project.org/wiki/index.php/Addon:PlaceCleanupGramplet
It works quite well, although it might add more information than you really want. It’s best run from the side bar, and you need a geonames account for it. The German name is a bit weird: Orte Bereinigung.