I have several 2960 and 3900 that have had an intermittent issue since firmware 1.4, and when I say a few I mean ten or more.

The problem is that after a while the router will not authenticate any login attempts via any method, this includes VPN and web front end, the router needs to be restarted in order to fix the problem, only for the issue to reoccur a week or so later.

It seems that the issue is related to OpenVPN, as although intermittent it seems to be only on units that are using OpenVPN connections.

Rolling back to pre 1.4 isn’t an option as the VPN offerings before then are very week once you rule our PPTP for security reasons, and 1.4.1 has the same issue.

Draytek support (UK) are not much help, they have said that there is not a know issue but this seems unlikely as I have the same problem on ten units!

Right now I am scheduling a nightly reboot to fix the issue but of course this is not ideal!

Is anyone else having the same problem?

I also did experience these problems with 2 3900 units after about 5 days uptime at the same day, during a period of two weeks in the beginning of october.
Disabled openVPN. The problems went away.
Upgraded both to 1.4.1 and enabled openVPN on one of the two 3900s. Until now, both run stable.