error messages regarding files with the extensions .ffs_db oder .ffs_tmp

[HansHans]

I used FreeFileSync for a few months now and it worked great.
One part of the syncronisation is the local PC working with Windows 10 Pro ,
the other one a distant fileserver is communicating through the SMB protocol.
This was the setting also before the error messages occured.

Now, suddenly, within the same network configuration, with the same files and directories, I get multiple error messages regarding files with the extensions .ffs_db oder .ffs_tmp

What is the cause?
What does it mean to me?
Can/Should I avoid them?

======================================================
error protocol

16:54:08 Error The file "Z:\ .... \sync.ffs_db" cannot be opened.
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [CreateFile].
Temporary access error: The file attributes of "Z:\ .... \sync.ffs_db" cannot be read.
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [GetFileAttributes.]
16:54:20 Warning The Recycle Bin is not supported by the following folders. Deleted or overwritten files will not be recoverable:
Z:\Elements\__Finance
Z:\Elements\__Self__
16:55:40 Error The file "Z:\...\sync.ffs_db" cannot be opened.
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [CreateFile].
Temporary access error: The file attributes of "Z:\...\sync.ffs_db" cannot be read.
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [GetFileAttributes.]
16:55:45 Error The file "Z:\....filename~ffs_tmp" cannot be deleted.
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [DeleteFile].
Temporary access error: unable to read file attributes of "Z:\filename~.ffs_tmp".
ERROR_FILE_NOT_FOUND: The system cannot find the specified file. [GetFileAttributes.]

[HansHans]

After the syncronisation, I tried another comparison of the two file systems without any error message now.