Hey sumwale, ran into this today. When uploading multiple files to a destination for the first time, it fails with an rsync syntax error (bare rsync works though).
mprsync --archive --info=progress2 file1.txt file2.txt user@destination.com:~/destination/path/
fails with rsync error: syntax or usage error (code 1) at options.c(2470) [client=3.4.1] printed over and over.
But doing just rsync --archive --info=progress2 file1.txt file2.txt user@destination.com:~/destination/path/ works. And if I do that once, and then run the original mprsync command above again, it'll work (probably because it's already uploaded and nothing else needs to be done).