After reassembling the sony-logo appears but thats it (the status-led is off but works, as in fastbootmode it turns blue). I came from Resurrection Remix® Lollipop v5.5.9.
I can access flash/fastboot and also tried to (not exactly in this order):
Did a full wipe
Do a System-Reset with PCC
Unlock bootloader again
Flash TWRP recovery.img with fastboot
Flash Stock-Rom with flashtool (1.236 custom de, downloaded via xperifirm)
Flash boot.img from CM via fastboot
Access TWRP and install Custom-Rom
Nothing helped, sometimes I only had the Sony-Logo, sometimes also the small xperia on bottom.
the phone is still "open", I checked all the connectors inside twice.