Hi,
See this thread for a workaround. This is caused by a new ASP.NET security update, it now throws this error if there is a postback with more than 1000 form elements. If you have a lot of roles then the checkbox lists for permissions on the feature instance settings will produce too many form elements resulting in this error. You may also get this error saving site settings if you have a lot of roles.
The next release of mojoPortal should ship on Thursday and will have a better solution, if the number of roles is higher than 20 it will use separate pages for the permissions to avoid this problem, but the workaround can solve it for now.
Hope that helps,
Joe