Sorry, I am not too familar with UEFI "mode" so would be able to elaborate? UEFI is just the replacement of BIOS.
Is the UEFI up-to-date? (I want to say BIOS so bad!)
BIOS-Version is 1900
It is a nightmare to get it to work. It sounds like you have managed to format and install Win7 right? Or I assume that you have managed the format part with the correct partitioning (and you have to have the UEFI Boot enabled when doing the install). If that is the case you have to re-work you Win7 installation disk. Apparently there is something wrong with the installation disk and you have to compile a new installation disk from your original. Try googling it, I can´t remember where I found the last clue, but lets put it this way. It was not worth it! I spent probably at least 2h getting it all setup and finally working, and to be honest I see no improvement whatsoever! It only creates issues. If you ever boot something else, like a USB thumbdrive or a rescue disc or whatever you have to go into BIOS and disable UEFI boot or it wont accept the devices / discs and so on.
I actually found the forum that helped the solve it:
Post #2 in that forum.
Just don´t waste your precious time on geek features that add no value...