I thought I still could use the old ECM-NV1 mic from our old A1E together with our new-bought PMW-100 without problems. This mic was also delivered with e.g. the PD-170. XLR=XLR and Sound=Sound would one say. Though, it is giving me a hard time now. I do get decent sound from it with the A1E but on the PMW-100 it keeps giving only very weak audio, neverless which gainsetting I'm using. What am I missing? Or is it just impossible to get this old mic working on a modern camcorder like the PMW-100?

Have you checked that it may require Phantom Power? (+48v) If there's an internal battery, it may be worthwhile to check that too. Some mics don't need batteries and can run on Phantom power and some can't and rely on battery power.

That mic is phantom power and has no internal battery. While I don't use it anymore and actually have 2 or 3 of them laying around taking up space, I did use them last year for a particualr job where all I needed them to do was pick up crowd noise so I had to run them to a phantom power box (a Fat Boy) and they did the job I needed for that job. Not exactly my favorite mic and I did notice something which I had forgotten and that is the levels are a lot lower than my AT897 and my AKG Blueline Hypercaroid.
Just make sure you go with Phantom Power and you might need to kick up the levels in your camera.

This mic has a very low sensitivity, I stopped using it long time ago, replaced with Rode NTG-1...