Horde IMP Unable to Retrain as Spam

I use Horde and IMP for web access to my mail server, and I have IMP configured to use the 'Report as Spam' link as follows:

/usr/bin/dspam --process --user %u --class=spam --source=error

 This worked great, as it prevented me (and other users) from having to visit the Dspam web interface to retrain missed spam.

Then one day, it stopped working.  The logs on the mail server would show this whenever the command above was executed:

[dspam]: Signature retrieval for '1,439c664a263161080180780' failed
[dspam]: Unable to find a valid signature. Aborting.
[dspam]: process_message returned error -5. dropping message.

After months of tinkering, I found the solution. I had to add Broken lineStripping to dspam.conf.  I can't explain why this solves the problem, nor can I explain why it was necessary all of a sudden (I never needed this before).