I have been running WSJT-X using a Kenwood rig with WSJT-X and full CAT control for months without trouble.
Now I am trying an Elecraft K2 with KIO2 and KSB2 options. I am running WSJT-X with aUSB/serial adapter connecting my PC to the KIO2 for rig control. I havea male-male 1/8" audio cable connecting the K2 headphone jack to the PCmicrophone input. With this arrangement, I can receive FT8 well and the CAT works fine.
I then have connected another male-male 1/8" audio cable from the PCheadphones connector to the K2 microphone input on the K2 front panel. Imake this K2 connection using a Y-cable with an 8-pin micconnector on one end and 1/8" female connector and 1/4" female connectorfor a PTT footswitch (not used for FT8). This is actually an accessorycable for Heil headsets which works fine on the K2 for voice work. The second I plug in the audio cable to the K2 microphone Y-cable, Iimmediately lose synch and the RS-232 connection fails.
When the connection fails, I get the following error:
/Hamlib error: Protocol error//
//serial.c(884):ser_close return(0)//
//iofunc.c(361):port_close return(0)//
//rig.c(1083):ser_close return(0)//
//serial.c(884):ser_close return(0)//
//iofunc.c(361):port_close return(0)  while opening connection to rig/
I thoughtincorrect audio level might be causing a problem so I added a couple of resistors per G4ILO to attenuate the PC headphone output to mic levels. No change in operation.
I don't understand why makingthe audio input connection would affect the rig control connection.I should note my PC has a combo audio jack so I must use a Y-adapter toaccess both the input and the output PC audio. This adapter works fine onFT8 with the Kenwood transceiver.
I've been reading old posts on this group but nothing has worked so far. 73, Murray VA1CQ

Joe,
I used the Kenwood strapping on my Front Panel board in my K2.
Pin 1: AF
Pin 2: PTT
GND: 7&8.
No bias resistor is required for a Kenwood mic.
I never added any capacitor on the line from the PC audio to my mic input. But the SSB adapter schematic shows a 2.2 uF capacitor on the MIC AF input to the board routing into the Speech Compressor. I have Speech Compressor configured OFF and Mic Gain set at minimum in the K2 menu.
Murray VA1CQ