Exmerge log fills up entire disk space!

I have been running exmerge as a simple mailbox backup utility without a problem for several months. (WinNT4 Server SP6a, Exchange 5.5 SP4).
Now all of a sudden, each time exmerge runs, the exmerge.log file fills up the entire free disk space of any drive it is on. I have changed drives several times with the same effect. The largest log file size was 19.3GB!! I did not try to open the log file as it was so large.
Logging level is set to 0 in exmerge.ini.

What is going on?