Background:

1. A Synology NAS is connected two networks, N1 and N2, both are able to reach Domain X.
2. The NAS was joined Domain X via N1. Share folders access is configured for domain users.
3. The NAS was disconnected from N1 recently.

Issue:
Users reported shared folder is inaccessible due to "Network Error".

Actions(Done within N2):

1. Verify NAS OS status and test shared folder access. OS is normal but shared folder can't be accessed.
2. Restart SMB service then following by restarting NAS. Issue persists.
3. Check default gateway and domain status. Noticed that domain was unavailable. Issue is rectified after rejoining Domain X with N2.

Conclusion:
Domain availability will cause shared folder access issue due to authentication, possibly.

I also noticed the same error, which appears to be caused by a time synchronization issue. In this case, the NAS NTP server was invalid, leading to a time discrepancy of more than 5 minutes between the domain and the NAS itself.