Yes, that does the trick! Confirming the "blue dialog" blindly brought me a step further. In addition i had to disable CSM (Compatibility Support Module) in my BIOS and now i can boot fine from Hard Disk and USB with SecureBoot enabled. Thanks for all your help. But what do you mean causes this issue: is it a poor UEFI implementation or has it something to do with my not so common screen resolution (1440x900)?