NAS full due to hidden files

I use my NAS and WDsync to sync my folders between multiple computers.

After 2 years of use, I observed that my 2 * 4TB NAS was full, while theoretically I only had about 160GB of files.
I eventually noticed that this was from hidden files, located in the WDsyncHistory of each folder (I was able to see them in my windows explorer by connecting directly to the networked NAS).
I erased all these files (it took me 2 days because there were almost 4 TB of them, hidden in about 20 folders) and thus found a “healthy” NAS, which only contained my 160 GB of files.

But they reappear at full speed!

These hidden files are unreadable, with no extension, I don’t know where they came from. They take up a huge space, and grow bigger every day: a folder containing 3.8 GB of “real” files, yesterday contained 134 GB of hidden files, and 163 GB today!

Note that I mainly store word, excel, powerpoint files, nothing huge anyway.

Does anyone know what these files are, how to prevent them from growing like this without doing anything (+ 30GB in 1 night!)?

Thank you in advance


You can refer to the following KBA: