JS8Call is not something I use very often, but I'm pretty sure it used to work with the TX-500. In fact, I wrote a settings document for the digital modes, with screenshots of all my settings, including those for JS8Call!
Background: FLrig is the most current version. It works perfectly with the TX-500. It works perfectly with all apps that can use Flrig as the rig control. The TX-500 is in DIG mode. The Digirig soundcar is v1.6 for the TX-500 and it works perfectly in all other digital apps.
However, now, when I start JS8Call after firing up FLrig, JS8Call reports "Rig Control error - Do you want to reconfigure the radio interface?" If I click OK, it shows the rig interface settings, which are configured as in the image below:

All the settings are correct, JS8Call is set to use Flrig and FLrig is working correctly.
If I click Test CAT, I get the following error:

I've tried using RTS and DTS and even setting the rig selection to Kenwood TS-2000 and avoiding FLrig entirely - Doing so immediately places the rig in TX mode and locks it there until I turn off the radio.
Any ideas what might be going on here?