There are several possibilities that could cause this.

If the problem is in parsing the GSV sentence, then one of the following fields must be bad: "message count", "message number", or "number of satellites in view". This seems unlikely.

If the problem is in parsing the RMC sentence, then probably your receiver does not attempt to provide any valid lat/lon data when the it does not have a fix. This is different from how my receiver works, which is to provide the "last valid lat/lon data" when there is no fix. If this is the case, then this is simply a matter of there being too many different NMEA implementations. I wish I had more GPS receiver models with which to test.

If anyone sees the "Invalid NMEA input from receiver!" message and has the ability to PM to me some NMEA output from their receiver, I can better debug Maemo Mapper's performance with their particular receiver.

The only reason we didn't see the error message before is because I didn't do any error checking before. Maybe I should revert to that behavior and hope for the best?