Firstly, this has to be the best error message I have ever seen.
We had an incident with a user creating a Plan and they got this fab error message. The issue seems to have resolved itself, but for future reference do you know what might trigger this particular one.
Thanks
Kelly
Back in 2014 we got this error in plans, inconsistently, when adding a primary worker to a plan and/or trying to save or create a plan with no primary worker. Restarting the Tess app pool was the suggested workaround at the time. The error has not recurred here recently.