The first error message means that Navisworks attempted to read in a given block of geometry from the file and that block failed to validate (most likely from an NWD). The second error message is a knock-on effect. We can't save out a new file with corrupt data, as we'd be saving out a corrupt file. The existing NWF can be saved because the specific block that is corrupt isn't saved into the NWF (it comes from a file that the NWF is referencing, and the NWF would still successfully save that reference).
Is drive N mapped to a network location? Do you get the same error if you try the same full set of files from your local hard disk?
There is a possibility that opening/saving over a network can yield corruption in translating the file between local memory & a remote location. We've had cases where opening & saving NWDs transfers the data in a different manner vs just copying the file & in that case the data can be corrupted during the network transfer by the time it reaches the local PC.