I had the same issue in the past. I removed flash player and installed the PPAPI flash player again. Keep Vivaldi closed until it's installed. Then it worked for me.I tried vivaldi yesterday under Shadow Defender.
I installed flash player for chromium but when i wanted to watch a movie i got a message <player not found>.
Except that when i went to adobe site to download flash player a message was telling me that i had flash installed but it is disabled.
I went to vivaldi plugins but no flash wasn't installed to enable it.
Does anyone knows what this is?