The reports I was getting to my mail said "Diskstation experienced an improper shutdown" although I'm 100% sure there were not any shutdowns (checked it later on).
Restarting the DiskStation by holding the power button and pressing it again once shut was bringing the server back online.
First I was thinking that these are hardware issues, but then I noticed that once I bring it back online and leave it for half an hour it would hang again.
So I figured there is something to do with the hibernation.
Tried different power settings, and finally got it to enter hibernation like it used to (status light flashing green, disk lights off) by disabling Advanced hibernation option.
However, the server wouldn't wake up from this hibernation too (it used to wake up by simply accessing it from the network).
Tried to reset (the full reset, including reinstall DSM) - didn't help, still having the same issue.
Disabling hibernation at all helps, but I wouldn't say I'm happy with that solution.
To sum it up:
- Advanced hibernation enabled - the server hangs with lights on, the mail report says "Improper shutdown", no access from the network.
- Advanced hibernation disabled - the server enters hibernation, status light flashing green, disk lights off, no access from the network, won't wake up.
- Hard restart brings the server back online, but after a certain period of time (I assume hibernation timeout) hangs again.
- Hibernation disabled - everything works fine.
- System reset doesn't help.
Has anyone encountered such a behavior and can advise?
For Synology tech support: opened a ticket #996010.