Support for SermonSpeaker 4.x is canceled as it is for Joomla 2.5. I will gladly assist you with minor issues but I will not fix any bugs anymore in this releases.
The fix is to upgrade to SermonSpeaker 5.x and Joomla 3.x, which is better anyway.
I saw this happening rarely and couldn't track it down. Most likely it's a Joomla issue of some kind as SermonSpeaker uses standard Joomla functions for the token checking.
The token is generated by Joomla in the form and checked again by Joomla after something is submitted. It's somehow based on the session of a user, but I don't know exactly how it works.
It's a security feature to prevent cross script attacks or so.