Could it be I somehow tell it to prepare to receive x bytes and only send x-1 or something?
Have to check..
Have checked:
If I fake numbers and announce +1 more character than I really send, the ESP8266 waits for a long time without saying anything. If I announce -1 character than I actually send it has no complaints, but now of course I have an extra character in the out buffer that will be taken as the first character in the next AT command.