When using different Assembly Versions of the Telerik.Web.Ui.dll in the Global Assembly Cache, the following or similiar error throws in the logs:
Telerik.Web.UI.RadFormDecorator with ID='RadFormDecorator1' was unable to find an embedded skin with the name 'Metro'. Please, make sure that the skin name is spelled correctly and that you have added a reference to the Telerik.Web.UI.Skins.dll assembly in your project. If you want to use a custom skin, set EnableEmbeddedSkins=false.
The used versions are Version 2013.2.611.45 by CalendarSummary WebPart and the Version 2015.1.225.45 used by Collaboration Manager.
Manipulate the web.config of all needed webapplications and servers in the farm to redirect the old assembly to the new one:
<assemblyIdentity name="Telerik.Web.UI" publicKeyToken="121fae78165ba3d4" />
<bindingRedirect oldVersion="184.108.40.206-2015.1.225.45" newVersion="2015.1.225.45" />
Run an IISRESET /NOFORCE and the problem should be gone!