Yes and no. The way I reported the issue and tested various solutions was wrong because I was looking at the same file on two different shares without noticing it. But (and I think I have not yet reported this part of the story) when I discovered the mistake, the first thing I did was just delete the file (the one that had actually been causing the error all along). This deletion worked fine, which means that I had full access to that file but duplicati nevertheless did not get access.
When I realized that this was equivalent to the original problem, I thought I might aswell check if it gets solved by changing the access rights, this time on the correct file. So I undeleted the file (moved it from the recyle bin back to where it was), changed the permisions using
What happened thereafter is reported here:
In other words, it worked as expected. But what remains unexplained, why I was able to delete that file before changing the access rights while duplicati was not able to even read it.
I realize that this may be very much of an edge case, so that I don’t blame you if you just leave it and attend to more important stuff.