I had a similar problem with a different configuration (Flex-6300, DAX audio, Dell PC). I was finally successful in what appeared to be a permissions problem. After a number of reinstalls and retries, I could see there was audio streaming on the DAX input channel on the PC, but could not connect that DAX channel to WSJT-X. I received the same error you did. I decided to see if I could connect that DAX channel to another program successfully. I chose Microsoft Voice Recorder which records audio from the default recording device and plays it back to the default playback device.
In Control Panel >> Sound I discovered my default audio recording and playback devices had been set to DAX channels which was not correct. I set the default playback device back to my PC speakers and the default recording device to the DAX audio channel. I then brought up the Windows 10 Voice Recorder to see if I could record the radio audio and then play it back over my speakers. When I opened Voice Recorder, it said it didn’t have permission to access my default recording device. I had to go to Windows Settings >> Privacy >> Microphone and turn on access permission generally and then specifically to the Voice Recorder program. Once this was done, I returned to the Voice Recorder, recorded some audio and played it back. The audio was from my radio, so I knew the path from my radio to a program running on my PC was working.
I shutdown / powered up my PC, and tried again to connect WSJT-X to the radio audio channel. This time, the connection was successful and WSJT-X seems to play successfully again.
Side note 1: In the process of updating Windows 10 through its multiple reboot cycles, Windows failed to access a USB drive I had plugged in. The only way I could get Windows to recognize the USB drive was to unplug the USB device, power down (shutdown) the PC, power up the PC, and insert the USB drive. From then on the USB drive is recognized.
Side note 2: In the entire process of resolving these problems, I uninstalled / reinstalled WSJT-X and my SmartSDR (radio control) software a number of times. Each time I did, I used power down (shutdown) / power up instead of reboot. Historically, Windows PCs have “remembered” too much (especially peripheral and network connections) during reboot that it needed to forget.Shutdown / power up resolves this problem. Use the software shutdown option to power off, do not simply turn off the PC.