I'm running STfax 4.5 (registered) here 24/7 mainly for answerphone & voicemail, but also for sending and receiving faxes. OS3.9 on 68060.
Always runs fine for me, never had any problems at all.
As it's MUI based, perhaps it might be an idea to investigate your MUI installation? A bad custom class could be causing all the grief.
Download and follow the instructions in my MUI archive, which you can find
here.
