ok finally I managed to make it work without any random crash.
I'm talking about the new ehci module with interrupts that I developed with rodries in june/july.
It has been abandoned since games crashed after playing for some minutes and we were not able to fix it at that time.
But I recently found out that the ehci module works properly after removing some non essential modules picked at random.
OMFG how many nights wasted in vain in hunting a bug that does not exist.
In fact this likely means that there are too many threads and problems arise when extending the execution time of the interrupt procedure.
So I removed the fat module by embedding fat support directly into the fs plugin, i.e. one less thread.
Also, I replaced the thread which makes the drive led blink on fat write access with a synchronous procedure which turns the led on and off respectively before and after writing sectors.
And now without those 2 threads the issue has disappeared.