If this were ripv1, I would say the router with these invalid routes has an interface configured with a different mask than the other routers--a /25, specifically. RIP normally takes the prefix length from the interface through which the route is learned, since the subnet mask isn't carried in the packets.
However, this is ripv2, so this shouldn't be happeneing--unless that one router is misconfigured with ripv1, or there are some ripv1 routes making it into the network someplace or another. What I'd do is check all of the subnet masks on all the interfaces of that one router, and then check the routing information source for each of those odd routes--the next hop in the routing table. I'd then pop over to the router that seems to be advertising a disproportionate number of them, and see how it's configured, etc.
Without much more information than this, it's going to be harder to give more than a general suggestion on how to troubleshoot it. If you don't find it at first, could you post a show ip route and a show ip rip database for one or two of these routes?
:-)
Russ.W
