Miro and Douglas, thank you very much for checking!
Your suspicion was right. Obviously i was not waiting long enough (things are really slow in trace mode). It continues and then goes to another loop, which is much more complex. If it is not a DSP problem it might be a problem on the host side (probably a timing problem?). I appended another trace output. But i'm not sure it is useful.
I found the problem. It is a timing issue. My system is too slow to run 68k + DSP in realtime. While the DSP is running performance of the 68040 drops to about 20 % in debugging mode and 40 % in normal mode. I recognized that for some reason the program does not fail on the 68030. The 68030 performance does not drop that far (around 70 %). So finally the problem seems to be this: While performance of the system drops to a fraction of normal speed, the timers (hardclock, event counter) are bound to host time and continue running at normal speed. That causes a situation where the frequency of DSP interrupts gets too high for the system to manage (after servicing an interrupt the next one is immediately following) and ultimately freezes the system.