This comment is for other users that might read this who have the same problem(s).
1) The (CRT) monitor, connected to the DVI output, is not detected at all.
2) The EDID is garbled.
These are two seperate problems. The first problem stems from the fact that a CRT connected to the DVI plug can be seens as being a DFP. You can solve this by using the Option "ConnectedMonitor" "CRT-1" (The '1' means the DVI plug for the card that I own).
An additional problem was that, because the EDID was garbled, the above solution did not work with driver 1.0-8776. It does work with driver 1.0-9631 however.
Nevertheless, in order to solve the garbled EDID (which is the result of the driver thinking that it's EDID version 2.0, while it's not) is to use Option "CustomEDID".
The following Device section gives me a perfect result:
# The DVI output plug on the FX5200 card with the VMP17
# (IIyama Vision Master Pro17 - MT-9017T) connected (using a DVI->VGA
# adaptor). The VMP17 gives a wrong EDID header, therefore the EDID is
# replaced with one from the file VMP17edid.bin. Moreover, this EDID
# (apart from the header directly from the monitor) contains wrong
# frequencies (too low), therefore also EdidFreqs and EdidMaxPClkCheck
# are being ignored.
Option "IgnoreDisplayDevices" "DFP, TV"
Option "ConnectedMonitor" "CRT-1"
Option "CustomEDID" "CRT-1:/etc/X11/VMP17edid.bin"
Option "ModeValidation" "CRT-1: NoEdidMaxPClkCheck"
Option "HorizSync" "CRT-1: 27-96"
Option "VertRefresh" "CRT-1: 50-160"
The used file /etc/X11/VMP17edid.bin is attached as VMP17edid.bin.gz and is the same as what my IIyama Vision Master Pro17 sends to the card when connected to the VGA plug (see startx -- -logverbose 6) but with the first 8 bytes replaced with "00 ff ff ff ff ff ff 00" and byte 128 replaced to fix the checksum.
Now going to get TwinView working again,