Thx for your answer. Unfortunately, it didn't help us yet in finding a fix for the problem.
The first IRQs are received after the call to pci_enable_device, but before the IRQ handler itself is registered (register_irq). The IRQs seem to be triggered by the write operations to the card registers (byteout()/outb() calls around lines 676ff in sedlbauer.c); the IRQ handler is registered much later, though. The dump above suggests that the kernel relays the interrupts to the USB handler (on the same IRQ) instead - which obviously is wrong.
In short:
* pci_enable_device
* writing to card registers -> triggering interrupts
(handler not yet installed -> error)
* register interrupt -> fails, as it already was disabled due to the errors
Again: thx for your comments. Do you have any further ideas/advice on how to get the driver running?