In JAMS, users may encounter the "Unknown Error" dialog, accompanied by the summary Default Error Handler, as shown in the below screenshot:

What does it mean?

The "Default Error Handler" message is used if JAMS does not have a standardized message for the exception. 

What is the real error, and where can it be found?

As "Default Error Handler" is used in cases where a message has not been configured, the nature of the error may vary. 

To find the true error, users will need the Stack Trace. This can be found in the JAMSServer.log file, located at C:\Program Files\MVPSI\JAMS\Scheduler\ by default.

NOTE: If submitting a support ticket for a "Default Error Handler" error, include a copy of the JAMSServer.log file. The location can also change based upon your installation

In the JAMS Server log, search for "DefaultErrorHandler" (omitting the quotes). The cause of the error can be found just after this line of text, as seen in the example below.
(Your actual error will likely not be the same as the one shown below)