Ulrik,
One of the first things I do is use BM Media Express program to test inputs and outputs.
you have to be very aware of what your input feed is i.e. 1280x720 59.94p
so if you are going to output and loop back (which I have done) what is the output setting in External portion of settings panel.
then you must match them exactly for input.
I have a great deal of experience with BM cards and 99.9% of the time it comes down to settings.
For example most people will select the 59.94i or 29.97p in frame rate for a NTSC 1280x720 input then wonder why it's not working.
What they overlooked was "i" next to 59.94 and what they should have selected was the 59.94p
Even if you have a camera set to record one resolution it may output a different resolution and frame rate.
A good example. cameras set to 720 @ 60p, actually output 720 @ 59.94p (drop frame)
I have clients call me almost daily just for this reason.
Use Media Exprees to see what's coming in. Again you will have to set the prefrences to match the source and you will need to use BM control panel to set type of input or output for Media Exprees.
Side note: don't try to run vMix at the same time you run Media Exprees, they both can't share the cards at same time.
Hope this helps.