It has been seen to occur when the Prognosis EMS collector cannot access one of the data sources defined in the EVENT configuration. Either it did not exist or it was blocked by security.
There may be another message in WVLOG from STEMS that occurs immediately after starting the EVENT configuration or restarting Prognosis that should identify which data source it is that it is having trouble connecting to. But if not then may need to enable trace on STPATH and STEMS collectors to capture trace when the error occurs.
The resolution was to identify the the offending data source in the EVENT configuration, comment it out or fix the issue with the security for that collector.