Okay, switching to English for sustainability 
I’m not sure what might be laughable about this problem.
Actually nobody seems to use Merkaartor from Debian anymore - the 0.20.0 package they provide in their repo seems to be build in an “experimental” context (and, no surprise, it won’t install on “stable”) as even Debian Package Tracking System - merkaartor only lists 0.19.0 for “sid” (=“unstable”, closest to what next “stable” = “trixie” might be).
Nevertheless, I was able to naively “backport” 0.20.0+ds-1 to Bookworm (“stable”), and there’s hope that it can be even backported to Bullseye (“oldstable”) which is awhat I’m trying now.
If I succeed I’ll have to open a bug report and ask for an official backport as 0.19.0 is now broken beyond repair, I guess…
Thanks for leading / pushing me the right way.
BTW, I had to tweak the “adjusted” setting in the Data tab: the URL must be “https://www.openstreetmap.org” (not “https://api.opemstreetmap.org:80” as after the “helpful” conversion). Only then I got an “Authenticated” report.
- S