Server loses default route

One of my servers was running Routing and Remote Access fine until a few
days ago. The only thing changed was that I installed some of the most
recent updates.

As of the moment it loses the default address every now and then. It has RIP
setup together with another router in my network. I added a static route to
the other router (which is the main gateway to the internet) so it's working
again, but don't understand why it's not working as before anymore. Before
it was all "automatic".

Any clues?