Random big dark blotches with "rays" on lightmaps

Hi! So for some time now I’ve sometimes been getting these weird big black blotches/spots (with “rays” I mean like those “arms” or whatever, making it somehow liik like a corruption that’s spreading, lol)
when I use baked/mixed (not realtime) lights (see image of lightmap below) (apologies if this has been addressed already somewhere, I couldn’t find these specific dark spots)

I had this happen in various projects already, in one case it were a bunch of somewhat delicate chairs who did this, I ended up disabling their shadow to “fix” this. I have tried cranking up the resolution to insane amounts, but that did nothing.

Apart from the chair example, usually these spots are rather big (this is like an area of 3 squaremetres)

It somehow looks like a bad blend between darker and lighter areas? I don’t know, couldn’t find a fix to this, not even the (to me) holy grail of deleting the graphic cache inside the preferences did anything.

Anyone knows what the deal is with these?

Thanks!

Check these visualization modes to see if you can spot any artifacts. Particularly the UV overlaps and texel validity modes. Unity - Manual: Debug Draw Modes for lighting might be worth posting some screenshots here