Windows 10 pro x64 22H2 with latest updates.
SyncBack Pro v10.2.129 (first noticed on v10.2.68)
Cryptomator v1.9.1 (first noticed on v1.8.0)
Neither SyncBack nor Cryptomator are running elevated as admin.
SyncBack profile source reads from a normal NTFS folder. The destination is a Cryptomator volume configured to use “WinFSP (Local Drive)”, mounted to a directory.
The SyncBack backup profile is configured to use compression (each individual file is compressed and destination file has .zipx added). Originally the problem was experienced using SyncBack’s “Fast Backup” mode, but it happens even in Mirror (and presumably any mode that has need to rescan the destination files on subsequent runs).
The first sync goes ahead without problems. On subsequent runs, if any destination directory contains hundreds of files, then the rescan goes into a loop and execution never gets any further.
Note that SyncBack profiles that do not use compression do not experience the problem.
My testing so far here seems to show the problem happens when the destination contains more than 320 files (starts somewhere between 310 and 320), but I guess it could be a buffer problem that is partly dependent on path lengths rather than just the number of files.
I first sent a problem description to 2BrightSparks (makers of SyncBack) and they studied the problem form their side. Their response was:
We observed that when requesting Windows to return the list of files in a directory it enters into an infinite loop without ever completing. Therefore, we suspect that the reading of the compressed file is somehow causing problems with your Cryptomator volume so it keeps resetting back to the beginning of the directory listing. There is no feasible solution within SyncBack to address this problem, SyncBack must open the Zip file in the destination to get the details.
And so here I am. 
It seems as if the particular combination of hundreds of files being scanned, and zip files being read during that scan, is causing problems for Cryptomator or WinFsp.
What next steps to try and solve this?
Background: This started as a way of getting encryption and compression for my backup. Cryptomator gives me encryption but not compression, SyncBack gives me compression (and it can do encryption but encrypting file names introduces other issues). Since I’m using compression I also use SyncBack’s “Fast Backup” which means it rarely needs to rescan the destination. And since all this works for most of my sources, it took a while before I hit the problem (reconfigured a profile that included some large directories, which meant it had to rescan the destination), and longer still to get to this point.
Typical folders where I am seeing this problem are generated source documentation (html file from doxygen etc.), and also subversion repositories.
