As there was no other solution to find, I flashed my MDA Pro to Windows Mobile 5 WWE - to be able to run Linux successfully on it. And it works now!
First I tried to run GPE with the newest Angström autobuild image with newest kernel available, but the kernels touchscreen support (and maybe other problems) and the image not starting an Xserver made me unable to gain a fresh experience. Finally I'm now running the lower one of Goboxlives GPE images for HTC Universal, which works relatively nice (the upper one I tried before seemed to be faster, but hadn't any phone support) since I removed an openmoko-theme (I don't remember the exact name) package, which made the whole UI look a bit weird. openmoko-dialer (of July 2007) is still working, but in Clearlooks Style :)
The phone capabilties are worse than QTopia (which is a great phone software, as I wrote before), it's impossible to receive calls and ending calls is difficult, but you have to take in account, that it is an 2.6.21-hh14 kernel (latest kernel image I could find yet is 2.6.21-hh20) and that this image is quite old (more than half a year). I don't know whether htc-linux project is already working on a 2.6.24 (would be nice, SDIO'd be possible and think of “Android”…) kernel for HTC Universal (website isn't as easy to use as openezx.org), but I think that there will be some newer images some day.
I think I'll try out some more images soon, e.g. Goboxlives' OpenMoko image which is relatively new, and I'll try to get Wifi and GPRS to work, as I didn't manage to that yet.
Edit: I just had a look at that Openmoko Image by Goboxlive. Well, I think I have to write on my view on OpenMoko later, but I first would like to use a newer image… Phoning with Openmoko seems to work, wifi does not (or I didn’t find out how it works). The UI is slow, but nice and usage.. well, you ‘ve got to get used to it, I suppose.