Pixhawk and uBlox M8N GPS

Hey guys!

Well I have an M8N GPS that didnt worked out of the box with my pixhawk.
I got no GPS signal at all.
So I used a USB<=>Serail bridge and connected it directly to the computer.
I used u-center and tested it.
With NMEA I get rock solid 3D fixes at all baudrates (tested 38400-115200).
See here:
https://www.youtube.com/watch?v=VypfPqO1P1k

Now that I know that the module itself is working correctly, I tested the ubx protocol.
Sadly this is not stable at any baudrates and measurement periods.
See here:
https://www.youtube.com/watch?v=y2WIN1sDLnE

I then used Mission Planner 1.3.39 and the GPS module connected to pixhawk and get an 3D fix.
But its loses the fix at least one time in a minute.
See here at sec 18:
https://www.youtube.com/watch?v=6GfosbBjnAg

I then took a config file for the M8N that I found here: http://ardupilot.org/copter/docs/common-ublox-gps.html
As soon as I upload the config file via u-center I dont see any signals in u-center.
I read that this is normal.
The bad thing is, that it also loses the GPS signal in Mission Planner.

I took a look at the "Full Parameter List" in Mission Planner and saw, that I can change the GPS protocol to NMEA.
As this gave me stable and good signals in u-center, I configured the M8N again in u-center to the out protocol NMEA with a baudrate of 38400 at 5Hz measurements, Airborne 4g and EGNOS.
This gives me good signals in u-center, but no signals connected to the Pixhawk in Mission Planner.

So here are my questions:
1. As NMEA protocol is working stable, am I right to say that my M8N is working ok or is it broken/defective?
2. Is there a way to let Mission Planner/Pixhawk work with the NMEA protocol?
3. How can I get stable GPS/Glonass signals on my pixhawk with the ubx protocol?

I really did a lot of reading on many forums, guides and did a lot of try and error, but I cant get it working on my Pixhawk.
I hope you can help me.

Ah by the way:
4. On some configurations I tested, I get a gpsstatus 4 in Mission Planner.
What does this mean?
5. With "my" an the "ardupilot.org" ubx cofiguration, I get "3D fix". I also tested a different config file and get a "3D dgps". What does this mean?

Thanks a lot!

The issue names no source file, test, or code entry point. Start by reviewing Mission Planner's GPS configuration and Pixhawk/uBlox protocol handling, then reproduce the reported NMEA and UBX behavior. Done would mean identifying the cause of the unstable or missing fix and documenting the meaning of the reported GPS statuses.

Written by the indexing model from the issue text.