Since upgrading we also have the problem that each night, at around 1 AM (1:05, 1:06, 1:08 are our last recorded times) something in the NAS happens and the system services stop working (WebUI, SNMP, SMB). Only SSH is working, but normal reboot is not possible. Just a power kill.
We will try a firmware downgrade to 5.2.4.3092 (currently backing up data in case anything goes wrong).