DevForce 5.2.1 -- which is an early release of the "DevForce 2009" generation of the product -- was built for Silverlight 3. DevForce 2009 is a legacy product, we don't sell it, and we encourage any customers still using it to upgrade to DevForce 2010. DevForce 2010, as of the 18.104.22.168 release, contains native support for both Silverlight 4 and Silverlight 5. It's a more robust product all around than DF2009, and it's not usually too hard to upgrade.
As for the error here - the version number of the System.Core assembly in both Silverlight 3 and 4 was 22.214.171.124, while it's now 126.96.36.199 in Silverlight 5. Some type shipping by SL, and type mapping by DevForce, does take place, so generally an application written for an earlier version may work. I would first check your XAP here to see which versions of the Silverlight assemblies are included. Also check the runtime version setting in the app manifest, and the minRuntimeVersion and autoUpgrade parameters on the Silverlight plugin object in the asp/htm page hosting the application.