I just managed to get it working, for some reason it appears that after running the unraid usb creation tool once it breaks something. heres is my experience:
1. every new usb i created through my own computer didn't boot ( except for the very first one a couple of months ago. )
i got a usb 2.0 usb drive and it still didn't work.
2. a friend of mine came over to have a good time with lan games this weekend and just for the sake of trying we created the unraid usb drive on his PC after which it immediatly worked no strange configs or anything through legacy bios not uefi. worked like a charm. so we switched back to the original usb 3.0 drive that i intended to use and made it bootable on his pc ( this was the second time using the unraid installer on his pc ) it hung on " syncing file system"
so we downloaded the unraid installer again and formatted the drive. still the same problem with syncing file system.
3. i tried a different pc which i had lying around and tried to make the usb 3.0 drive bootable on that machine and it again worked like a charm just like we tried it the first time on the pc of above mentioned friend. server boots whithout an issue and it is nou parity syncing.
conclusion:
i have read over 10 forum pages with possible solutions this weekend and in the end it appears to have been an issue with my pc or the program which creates unraid usbs and nothing with the server itself...?
i have no better explanation tor this strange situation but i am glad it is finally working after 3 days of trying.