Same for me, though started somewhat later. One suggestion here was to rename the default (profile) folder, so a new would be created. That didn't work for me.
I then downloaded the latest version and installed (it asked if I wanted to 'update', though it was latest version). Didn't work either.
Occasionally, it worked for me to launch in private mode and then open the normal Vivaldi, but not everytime.
Unless anyone has a better idea, I guess I'll have to uninstall completely and start from scratch...
