error executing updater binary in zip

hi, i'm running cm 12, twrp 2.7.0.0, and when i wanted to flash cm 12.1 i got the following error:
"error executing updater binary in zip"

i tried:
1. wipe
2. updating to twrp 2.8.7 (and then came back to 2.7.0.0)
3. installing few cm nightlies

well, nothing worked. iv'e read a lot on line, and what i did came from there, the only thing i didnt try is to flash a different bootloader because i couldn't find a straight forward answer what to install (and i'm scared to brick the damn thing)

what should I do?
THANK YOU!!!!!!