Had the same problem. All 3 windows 7/10 machines. 2 computers with external hardrives plugged into USB were fine. Computer on LAN with NAS said not enough hardrive space even though there was over 3TB. I downgraded one with NAS to 2.5.13 and it APPEARS to have solved the problem. BTW, this problem appeared overnight during christmas break. No issues for months and months, then all of a sudden two folders would not sync.