I upgraded my DNN 4.6.2 site to 4.9.5 this morning, www.oaklandsymphonychorus.org.  Everything went fine, I tweaked a couple of configuration items that needed it, then went to lunch.  After lunch, I went to update a notice - on a different computer - and after clicking on the LOGIN button I got this error:

A critical error has occurred.

Failed to load view_state. The control tree into which view_state is being loaded must match the control tree that was used to save view_state during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.

I googled the error and haven't found anything relevant to a 4.9.5 site with only minor customizations.  I logged in as host and restarted the application.  I haven't seen it again, but my user base is very non-technical and I really don't want them to see this.  What did I do wrong??  What do I need to do to fix it??