FreeFileSync Forum

Unfortunately aalex24 does not mention on what operating system the error occurs. There is another discussion in https://freefilesync.org/forum/viewtopic.php?t=7265. Following a change in my operating system (from LM Mate 19.3) to Ubuntu Mate 20.04, I had a problem with the number of inode watches ...

In my case it was before the actual change a Linux mint Mate 19.3, now Ubuntu Mate 20.04. Maybe there is a complete different reason for your problem. According to your path /opt/FreeFileSync... it seems it was installed as root? Do you have the right permissions to execute it as regular user?
What ...

It might be affected by changes in the operating system. I had difficulties to get RealTimeSync working on a newly installed Ubuntu Mate 20.04.
It depends on the number of available inode-watches.

To find out what is the maximum number of inode-watches, put in a terminal (as regular user):
cat ...