I tried to use the old 2.6.22 kernel instead of the new 2.6.24 one, but no chance. Xen did not show any error messages and started the DomUs, however neither networking nor "xm console" were working. In fact "xm console" did not show anything and did not react on keyboard input and even a SIGTERM caused no effect. I had to kill the instance with a SIGKILL or SIGHUP.
Now, after I have restored a backup of the old system, everything is working fine again. Therefore I advise everyone using Xen on Ubuntu not to upgrade to 8.04. Upgrading the DomUs however did not cause any problems, Ubuntu Hardy Heron seems to be broken for Dom0 only.