Yes, I have verified all very carefully and today I have also experimented with erros:
By putting intentionally the wrong toggle bit in the IN transfer-descriptor (toggel=0 instead of =1) and with the interrupts activated, I don't get an error, but simply nothing! That means an interrupt has captured the error. It must be an interrupt, not an event or break, since with disabled interrupts I can poll for the error normally.
I then re-vectored all protected mode ISRs to see where the interrupt comes from, just to see that still a real-mode ISR from the BIOS must be active! Since I have released EHCI from BIOS (in the LEG_EXT_CAP register) as prescribed, there must be still some other BIOS related register, I suppose. I've searched my chipset datasheet for "UHCI", but didn't find anything relevant.
Any ideas how to release the UHCIs definitely
Must there occure a reset and new setup of the PCI