Hi @pribis and welcome to the forum!
This might take some guessing. First, I’ll say it works fine for me, using a USB thumb drive formatted FAT32.
Would you happen to have one to see if it does the same thing? That seems an easy way to rule out ACLs.
Also note that if the original situation involved a pluggable drive, plug it in before adding its job, to show it…
I have a CD reader with a CD in it, which I can view in File Explorer, but Duplicati wisely (?) doesn’t show it.
So possibly there’s some kind of filtering going on, to avoid showing drive letters that can’t be written onto.
Did you happen to let it try to create the path? Or if that feels risky, let it make a folder next to the real one.
If you try it and get a failure, the specific problem that occurs during the creation attempt might give a clue.
Note the Windows drive letters are per-user. Differences may show up mostly on shares or subst (I tested).
The next best test I could try short of setting up a special pluggable drive was to remove permission from a
folder on my hard drive. I took it down to just me using right click and some fancy access control list work…
Got it in File Explorer, copied and pasted to Duplicati, which didn’t find it, and (when asked) couldn’t create.
I think the “does not exist” message might go off for other cases, such as “can’t even get there to find out.”