I fixed it by installing a "preview update" of Windows. Normally I don't install the preview updates, but this time it worked.

HD Homerun Prime with Xfinity cable card. Was working fine yesterday, now the Windows app crashes with this message "Null strings man not be marshaled in Windows Runtime arguments. Parameter name: sourceString" Android apps, no problem. Firestick app no problem. Google TV OS, no problem. Other ...