Ouch. Thanks for the elaborate corrections, @bsinou — it seems that I had a lot of misunderstandings, I do apologise for them. I jump too easily to conclusions based on incomplete data hehe 
But I really appreciate the explanations and corrections! I’m always learning new things…
In any case, aye, I can confirm that this is definitely related to my other issue: in other words, whenever there are folders which have been replaced with files with the exact name, Cells turns crazy, grabs all possible file/socket/resource handles, and does ‘something’ with the memory-mapped files which I could not figure out. But all these symptoms are clearly related: once I removed the structured storage and added it back again, everything started to work flawlessly again (I believe I had to restart Cells, but now I’m not even sure if that was necessary or not).
I’d guess it’s not even a bug in Cells — more likely a bug in the core Go libraries themselves, or even in the Go compiler. It’s hard to say; it’s certainly one of those nasty ‘edge conditions’ that will never happen to 99.99999% of all users, and then, one day, someone ‘decides’ to upload a file with the exact name as a folder, and that triggers all these mysterious symptoms.
In any case, with the same caveats as the other issue I reported, I’ll mark this as solved — the workaround works (sorry for the pleonasm!) and that’s enough for me!