This happens to me too. If I have a collection open, and attach a note to an item in the collection, then drag the attached note OFF the item and drop it into the main list for the collection, then I get that error next sync.
The workaround is to find the note and put it back into the item whence it came, or else delete the note (...and then empty the trash!)
(It only happens about half the time. I can't find a 100% reliable way to reproduce it.)
Thanks — I'll see if I can reproduce that. In the meantime, if you don't have build 166 yet, upgrade to that. It might not help, but it fixes a bug in the vicinity of this issue.
Upgrade Storage
There is a sync problem in Zotero 5 beta.
Report ID: 1466304906
Error name: Child items cannot be assigned to collections.
Thanks
The workaround is to find the note and put it back into the item whence it came, or else delete the note (...and then empty the trash!)
(It only happens about half the time. I can't find a 100% reliable way to reproduce it.)