Z3 rebooting after 40 seconds in Flashmode topic

Hello!
My Sony Xperia Z3 (D6603) is rebooting by itself 40 seconds after entering Flashmode so I am unable to flash a Firmware

Does anybody know how to fix this Problem?

I have Firmware 23.0.1.A.5.77 with German T-Mobile Branding and a locked
Bootloader (I don't want to unlock it)

Thanks to everybody in advance