Odd terminal behaviour - Wyse WY-55ES

Well, I'm missing something... Z80 machine configured for two users, two serial port, SIO1 & SIO2 With PuttY I can connect to either port. Escape sequences are not handled correctly (this has been debated in another thread ...). Wyse *real* terminal, same cable. Straight through, not null modem. I can connect correctly to port SIO1 (console). But I have to connect to the modem port of the terminal. 9600/8/N/1. I was unable to make it connect to the AUX port. Escape sequences are correctly handled. Can't connect to port SIO2. Same configuration, only the cable has been moved from a port to another. Well, but the odd thing is this. If I press SETUP on time, the login prompts pops up. If I press a key followed by the SETUP key, that key is transmitted and displayed. So the connection works only pressing SETUP after every (*every*) keystroke. Can't be the port, that cable works with putty. May be the wiring and Putty more tolerant?