I do not know, to be honest, what internal check DIAGNOSE failed.
It said it could not verify the I/O address. Maybe it tried to do DMA and this failed?
Regarding the IT8892/3 and the IT8888, I did some more research and from what I read in the technical details (manuals, whitepapers) of some dedicated industrial mobo makers (Advansys, Kontron) it is necessary/mandatory to set up the positively-decoded addresses/ranges in these chips to set them up for actual working ISA DMA.
The user must set this up in their BIOSes, these have according menus for that.
This also is consistent with what I read in the IT8888 data sheet register descriptions.
However, the DFI BIOS lacks any such settings.
So I assume that DFI's intention was that this setup has to be done by the user, for example by writing some small setup program that sets the chips registers as needed by the ISA hardware present. Or, maybe they expect(ed) their customers to hire their engineering staff to do this?
Thus I have changed my project path a bit. Right now I am working on trying to modify MACHINE.INF and MACHINE2.INF for proper setup of Windows 98 with the Series 8/CS 220 chipset, so that Windows installs with PCI bridge, so that PCI hardware gets detected correctly.
For this I have deactivated all of the mobo onboard hardware that can be deactivated, in favor of using a PCI-E Nvidia GeForce PCX 5750 (which I hope still works quite good on DOS graphics games), an Adaptec AUA-4000 (NEC USB 2.0 chips), an Intel EtherExpress 1000GT (one of the last Intel cards with Win98 drivers).
Regarding the sound card, I'll have to try a bit. The DFI board allegedly supports DDMA, and thus there is a small variety of some potentially usable sound cards to try out. I guess I'll start testing with the Aureal 8820 chip.
If I manage to do this, I'll use the board with this PCI DOS and Windows 98 compatible hardware, leaving the IT8888 programming for ISA soundblaster for a possible later project.
Even without ISA, I'll be satisfied then, as the mobo cost me 70 euros only, it is of good professional quality, not like consumer boards, and 4 PCI slots is rare on relatively modern mobos.