You are mostly likely using the first version of MacX25; the MacX25 server and MacX25 Admin software version 1.0 doesn't work on any computer that requires System Software 6.0.7 (such as the Macintosh IIsi). The problem involves the "Gestalt" function and is fixed in version 1.0.1 of MacX25.
Version 1.0.1 of MacX25 is compatible with both System 6.0.7 and System 7. To use the MacX25 server software under System 6.0.7, you'll need at least 2MB of RAM. Under System 7, you'll need at least 4MB of RAM.