When you upgrade to a new version of mojoPortal, you need to use the new Web.config file and transfer any customizations such as machinekey from your old one. You did not do this so your Web.config file is missing something that fixes this problem.
Backup your current Web.config file and install the new one from the mojoPortal 2.3.9.6 package and restore any customizations such as <machinekey from your old one to the new one
It will solve the problem.
Hope that helps,
Joe