We have a client that is currently on iMIS 15.1.1.3632. I have this first client's database on a dev server and upgraded their database to iMIS 15.1.2.5184 and was still able to recreate the error. Just today I received the same error on another client's iMIS Public site - they are on iMIS 15.1.2.5184.
Both versions are getting similar error messages. When a user goes into iMIS Public to register for an event - or to register another roster member for an event - they receive the following error message (this does not always happen, but it happens enough to where the first client that is currently live can't use event registration and our 2nd client won't go live without a fix for it):