Are you running the newest AppleShare client on the Macs? I'm not sure how far up you can go with System 6. In 7.1 I got an application error when trying to mount volumes served from my bootcamp partition (Windows XP running the Windows 2000 Services for Macintosh) using the System 7.1 AppleShare extension. I replaced it with the one from 7.5.3 and it mounted the shares just fine. However, while I could read files from the shares, I couldn't write any to them (I could in 7.5.3).

The issue is that there are different versions of AFP. According to Wikipedia, Services for Macintosh uses AFP 2.2 which is most likely incompatible with the client extensions in System 7.1 and earlier.

Incidentally, while the readme for AppleShare client 3.7.4 (bundled with OS 8.1) says it requires System 7.5.3 and up, it works fine here in 7.1 (I'm using OT).