I had the same exact issue. The fix is pretty simple. All you have to do is uninstall the software. Then go ahead and reinstall it normally through the play store. After the install the problem should be gone.
I had the problem where the phone, text and voicemail tabs of the google voice app all were blank and all had a pop-up saying "error loading conversations". Re-installing google voice didn't work. What caused the problem was that earlier in the day I was disabling apps I didn't use, including my phones contacts app (since I use google contacts). When I enabled my phone's contact app again, google voice worked again.