Have you tried these steps?

Install the Windows 98se Shutdown Supplement - Look for 239887UP.EXE here: https://www.computerhope.com/download/win98.htm

Disable fast shutdown https://smallvoid.com/article/win98-fast-reboot.html

Yes. That makes no difference unfortunatly. The shut down problem only exists when the 3COM card is present.

I know it's an old topic but I encountered just that and managed to solve it. The blame's on the IRQ set to "auto" in bios. That conflicts with another PCI board that you have installed and that uses the same IRQ, which this particular board does not like. I encountered the same problem and had to do a lot of troubleshooting to get there. Funny thing was that I had bad driver issues with the Sound Blaster because of that and that led me to think it was a conflict. I set the IRQ of the PCI slot in which the 905B was installed to 10 and the IRQ for the sound card to 9. They both ran fine afterwards. For whom it might concern...

I never had any problem with 3com cards.
Try a Fresh install of Win98 without the 3com card installed.
Make sure the computer works fine.
Run MSD and write down all the IRQ’s used by each device.
Install the 3com card and run the 3c509b diagnostic program.
Run auto configure and set to use Ethernet.
Use the Win98 drivers provided on Install CD