The 2.4.21 kernel only needs the nvnet module, not the audio or the ide driver to get the Nforce2 board working.
I unpacked the nforce tar file and changed directory to the nforce/nvnet subdirectory, do a make clean, make and then make install. Works everytime.
It may be a silly question but did you remember to do a "make clean" ? Seems to me that in one hurried afternoon, I forgot and did the install of nvnet from the last kernel and got the "unresolved symbols" issue.
If you didn't set the 2.4.21 kernel to support AMD74XX in the IDE, ATA ATAPI Block devices then the disk DMA doesn't work. This switch is shown in the 2.4.22 series as AMD and nVidia IDE support.