The client instance you refer to is an instance of Client, and its available() method is known to have issues.
Here is the other report:
Do you use Serial.Available() in your code (wild guess what could be the problem)
This was robtillaart in reply #1 of that topic.
Nothing returned from a card swiper would be 0xFF. It's all ascii text.
I know this is probably really unlikely, but is there any way to tell the arduino to invert the serial data it receives before interpreting it?
The rs232 signal is already being inverted.