I now tried the exclude statement you suggested, but that didn't help. As in the case when I used both the modem and the Ethernet card at the same time, the modem got 0xaf8/5 and didn't work. I also tried some other exclude statements to force the modem to other addresses like 0x2e8 etc., but I never got anything else than "autoconfig failed".
What does serial_cs do if autoconfig fails? Does it leave the card at the address it assigned before doing the autoconfig, or does it disable the card afterwards? Because I also tried using setserial to configure the card manually, but I only got I/O errors when trying to open the serial device. Of course, if serial_cs disables the card when autoconfig failes, this cannot work.
The parse errors from dump_cis still worry me. Could they cause these problems?