Smith007

So it actually works like a charm. All you have to do is to list files you want the mover to skip in a text file, and tell the Mover Tuning plugin to use it. In my case seems it was enough to list:
 
/mnt/cache/NEXTCLOUD/nextcloud.log  
(not sure, but I reckon it is important to use /mnt/cache/* and NOT /mnt/user/*)
 
Seems it solved the problem with triggering secondary storage, but need some more days to confirm.