Thats embarrasing… I saw that post, but didnt get its meaning at first…
Thanks for pointing me in the right direction!
There is some cross correlation, though, as this was not the only problem.
I found this thread, which lead me to believe that something is wrong with using a network drive, as a counter test revealed that getting a session token of a local folder was fine.
Basically now I am not referencing \\Server\[...], but rather beginning from the assigned network letter S:\[...].
And instead of fs.createSessionToken(await fs.getEntryWithUrl("folder:" + url)) I use fs.createSessionToken(await fs.getEntryWithUrl(url))