I've installed latest Composer 4 R2 onto a CMS 6 R2 website and created a content block. When I'm dragging the content block onto the Extension Content Area, I get this error message:

An error has occured
Could not load function. Please see log for details.    

The log is empty though (Level Warn).

When I add the standard Composer templates, I get errors like this instead:

System.FormatException: Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).
   at System.Guid.GuidResult.SetFailure(ParseFailureKind failure, String failureMessageID, Object failureMessageFormatArgument, String failureArgumentName, Exception innerException)
   at System.Guid.TryParseGuidWithNoStyle(String guidString, GuidResult& result)
   at System.Guid.TryParseGuid(String g, GuidStyles flags, GuidResult& result)
   at System.Guid..ctor(String g)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderExtensionPageData.Read3_ContentFunctionData(Boolean isNullable, Boolean checkType)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderExtensionPageData.Read4_ContentAreaData(Boolean isNullable, Boolean checkType)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderExtensionPageData.Read5_ExtensionPageData(Boolean isNullable, Boolean checkType)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderExtensionPageData.Read6_page()
2012-02-02 18:26:40,910 ERROR [34] Dropit.Extension.Core.Serializer.Deserialize - Dropit.Extension.Core.Serializer: XML to be deserialized: <page pid="4337" wid="16400"><ca id="MainArea"><cf pid="0" wid="0" typeid="0" typename="error" isglobal="true" guid="null" remotesite="null" IsPersonalization="null" vgcid="null"><a href=""></a></cf><cf pid="0" wid="0" typeid="0" typename="error" isglobal="true" guid="null" remotesite="null" IsPersonalization="null" vgcid="null"><a href=""></a></cf></ca></page>
2012-02-02 18:26:40,910 ERROR [34] Dropit.Extension.Core.Serializer.Deserialize - Dropit.Extension.Core.Serializer: XML to be deserialized: <page pid="4337" wid="16400"><ca id="MainArea"><cf pid="0" wid="0" typeid="0" typename="error" isglobal="true" guid="null" remotesite="null" IsPersonalization="null" vgcid="null"><a href=""></a></cf><cf pid="0" wid="0" typeid="0" typename="error" isglobal="true" guid="null" remotesite="null" IsPersonalization="null" vgcid="null"><a href=""></a></cf></ca></page>

I can trace the error message back to the js function "Extension.CallbackContentFunction.prototype.OnLoadAbort", but I can't find what casts that error.

For Composer 3.2 there seemed to be a problem with the web browser, but not in Composer 4.

Any suggestions on what to do? Any help is appreciated!