Skipped step "fails" with message "Library not registered."

[dmcnulty]

a "Send Mail" step in a skipped group failed a few days ago with a message in the log, "Library not registered." We have not been able to reproduce this error since. What does the "Library not registered" message mean, and how can we prevent this error in the future?

Attached is a screenshot. Here is an excerpt from the log:

5/29/2007 2:44:10 AM: Building project step 'Perform Catastrophic Notification'...
Step 'Perform Catastrophic Notification' build rule evaluates false: %LASTSTEP_STATUS% (expanded value = 0) is not equal to 0
5/29/2007 2:44:10 AM: Step skipped
5/29/2007 2:44:10 AM: Building project step 'Report build failure'...
Library not registered.
5/29/2007 2:44:11 AM: Step 'Report build failure' failed
5/29/2007 2:44:11 AM: Build ended.

Thanks,

_Dave