Unresolved symbols with mga_vid on 2.4

This is a bit of a head-scratcher.

I've been running 2.4.20 happily with mga_vid for months. I
recompiled the kernel to enable UDF writing. When I rebooted mga_vid
wouldn't load.
Naturally I recompiled mga_vid but I get the same result.

depmod -ae says:

depmod: *** Unresolved symbols in
 /lib/modules/2.4.20/kernel/drivers/video/matrox/mga_vid.o
depmod:         pci_scan
depmod:         map_phys_mem
depmod:         unmap_phys_mem
depmod:         _GLOBAL_OFFSET_TABLE_
depmod:         puts
depmod:         printf
depmod:         strerror
depmod:         mtrr_set_type

modprobe mga_vid says:

/lib/modules/2.4.20/kernel/drivers/video/matrox/mga_vid.o: couldn't
  find the kernel version the module was compiled for
/lib/modules/2.4.20/kernel/drivers/video/matrox/mga_vid.o: insmod
  /lib/modules/2.4.20/kernel/drivers/video/matrox/mga_vid.o failed
/lib/modules/2.4.20/kernel/drivers/video/matrox/mga_vid.o: insmod
  mga_vid failed

Can anyone suggest how I might fix this?

Martin