Thanks @sko for the hint.
Culprit found. It’s automount for me, too.
Disabled -> no problem logging in.
Enabled -> logging in halting at “finalizing” “forever”. At least too long for me to wait out.
But: This is only a problem with Lumina. No problem with xfce in this regard. And, as far as I can tell and remember, this didn’t happen with the pre-17.12 STABLE update and it doesn’t happen with the UNSTABLE “release”, at least not the one from about 2 weeks ago.
“Only” the dev “know” what happened to “automount” in 17.12 that wasn’t there before and was fixed in UNSTABLE.