I posted the same question on the Telerik Forum and they admitted that they could reproduce this error but could not find any workaround at the moment. They added the issue to their Lighswitch integration roadmap but the timeframe for the fix is yet unknown.
I solved the problem replacing Telerik OpencAccess ORM with classic WCF RIA Service solution (according to the tutorial: http://www.c-sharpcorner.com/UploadFile/raj1979/how-to-use-wcf-ria-service-in-lightswitch-2012/ ). I also had to switch to Visual Studio 2012 to be able to create a Domain Service Class.
Summing up, I had to change some tools but everything works perfectly now.