i found a solution for me. My Error message was "... file is locked by other application", but in fact the process user running the rslsync service had no write permitssions on the local folder.
The process user for me is 'rslsync':
rslsync 531 1 0 Jun27 ? 00:04:20 /usr/bin/rslsync --nodaemon --config /etc/rslsync.conf
Here the group has no write permissions which causes the error
host:~/folder$ ls -la
drwxr-xr-x 3 user rslsync 3488 27. Jun 12:02 .
drwxr-xr-x 5 user rslsync 3488 23. Jun 14:21 ..
... files ...
Solution for Linux/UNIX:
chmod -R g+w ~/folder
For Windows or any other device.
Check the process user and this permission on your local sync folder.