The issue is with AppleShare Client 3.8.3 which is installed with Mac OS 8.6. A workaround is to mount the volume before adding it in the Macintosh Manager. You could also run the Macintosh Manager on a Macintosh running Mac OS 8.5.x which has AppleShare Client 3.8.1 installed.
This issue will be resolved in a future version of the software.