Here are some solutions I've found to contribute Bernt.
Getting measurements in a stable manner with an ASUS Xonar U7 MkII and Umik.
Using ASIO as the playback device works fine on the Xonar U7 MKII. I use WASAPI as the input device. These are probably evident suggestions here are the ones that are not so much evident:
1. Lowering your sweep duration from 10 to 5 seconds definitely increases your chances of success.
2. Go into advanced settings and then turn WASAPI exclusive mode to enabled.
3. EVERY TIME YOU TAKE A MEASUREMENT turn clock drift correction on and off. It resets something in the software, I'm not sure what but when you turn it on and off the entire UI seems to flash for a second like something major has happened. I don't know what it is but I was able to take over 10 measurements without audiolense crashing by toggling this between measurements. I then tried a few measurements without toggling it and got an immediate 'file open' error so I believe this is a method of increasing stability.
Another thing I've noticed:
Under speaker setup as you go through the tabs from left to right from speakers to cross over configuration there are two places where you can set octave width. Make sure that when you change it in the cross over configuration tab that you return to the speakers tab to verify that it 'stuck' because often it will return to it's default 2 octave value so MAKE SURE that you check it before saving your speaker setup configuration.