OK, here's a solution that seems to work, at least for me:
Install the latest version of Macintosh Runtime for Java (MRJ, version 2.3.3, you can get it off apple.com) into the OS9 system you use in Classic. Using the OS9 version of Internet Explorer running in Classic, Digichat seems to be fully functional again.
Rudi and others, HTH.
Aloha.