Hi, Makgirl!
My lightmaps are clean and crisp:
Yes, the resolution is low, but it is optimal if you look in Lightmap Density mode. Lightmaps were not the issue, as it turned out. I will post the solution in a separate comment, so I could mark it as an answer for people, who might stumble upon the same issue.
However, I’m still having some troubles, and if you could maybe take a look at it, that’ll be great! I’ll post it in the next comment as well.