This error: 

Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. 

This means a security feature is enabled in your website primarily to protect against injection attacks that can be launched with post values.  So some controls that don't register their event for event validation may trigger the event. 
Microsoft added the security feature in asp.net 2.0 and DNN later made it a default setting. 

If you need to you can turn it off in the web.config of your DotNetNuke installation: 
<pages validateRequest="false" enableViewStateMac="true" enableEventValidation="false">

Turning off security features is not ideal so you could also contact the control/module vendor or whoever to have them update their control or if you're developing your own controls then use: 

ClientScriptManager.RegisterForEventValidation

Here's a good reference for more.