Cannot copy file error

[vqvq]

I've been using FFS for some time now without any issues. Suddenly, I'm getting a completely unknown (until now) error, e.g.: Cannot copy file "G:\data\abc.dsk" to "D:\data\abc~c89e.ffs_tmp" Error code: 5 Access denied [CopyFileEx]
Here, G: is a USB external HDD, D: is internal SSD in my desktop PC with no specific restrictions (not a system disk, I use it only for data). This error pops up for each file to be copied, FFS is therefore practically unusable now for me.
Also, after this error, and after closing FFS, I can not eject/disconnect the USB external disk, Windows says it is still being used. And again, this never happened before just after using FFS.
Note: The same behavior exists also after complete reboot of Windows.

Some (very recent) history: I use the donation edition 10.18, build 19.11.2019. Recently, after a Windows upgrade on my laptop FFS stopped working completely, so I have (1) uninstalled FFS, (2) installed the latest version 10.20 (not donation, my donation period expired), (3) did some syncing successfully, (4) only to face error with incompatible ffs_db files when syncing on my desktop PC running 10.18. So I have uninstalled FFS 10.20 from my laptop, installed FFS 10.18 and erased all ffs_db files to avoid that error of incompatible ffs_db. But now it's not working at all

What can be the reason of the error?
Many thanks,
Vaclav

[vqvq]

Some update:
I have tried to completely uninstall FFS, reinstalled the latest version. I created a new task to sync folder from disk D: and G: (the same drives described above) => All seems to work fine.
BUT opening the original .ffs_gui file, I get the same error "Cannot copy "....." to "D:....ffs_tmp" _ Access denied.
What can possibly be the reason?
Many thanks,
Vaclav

[vqvq]

Update 2:
I tried to run FFS with elevated permissions and now the copy operation works. Obviously, this is not something I would like to do as a norm.
Any hints welcome
Best regards,
Vaclav