FreeFileSync Forum

I get this error message very often (macOS Sierra 10.12.6):

Cannot write file "/Volumes/QNAP-1/Backup Documents/My Documents/Folder/Folder/Folder/file.pages/preview-micro.2104.ffs_tmp".

Error Code 21: Is a directory [write]

I'm logged in to my QNAP with admin rights. What's the problem?

Tnx ...