You have probably tried already but other baud rates?
9600, 1200, 2400, 57600
Seems like 8,N,1 is common?
8 DATA, No parity, and 1 STOP bit
Try changing what the Enter key does under the Terminal Keyboard setting might not be available in putty can't remember.
Try typing ESC[20h in putty after connecting to the device.
Try another terminal such as RealTerm?
Looks similar to what happened in this buspirate video on hidden serial ports, although his issue turned out to be wrong wires.
A full series here might have something useful to debug why this is happening?
Just thoughts that come to mind as it looked like something I saw elsewhere, however I don't have a lot of experience in this area yet and have only tampered with basics with the bus pirate, sorry in advanced if im not helping...