Sync don't work for documents folder

Hi everybody, I’m usining Nextcloud on OMV5 as cloud for my files.
Hi copied on the default documents folder all my files (PDF, DOC, XLS etc) and I created other folder where I copied other files (for my CNC, Printer 3d etc.).
Then I installed on my PC the Nextcloud APP ( I installed it on two PC with Windows 10 and on a Mac Airbook) but the syncronize of the only documents folder, on WIN and on Mac, don’t work, it say me “503 Service Unaviable” a “get h_t_t_p_s:/_/” (cloud not open file) in a red box.
The others folders, instead, syncronized perfectly.
How I can resolve this problem?
Thanks and ciao