My default search engine in Vivaldi reset to something called Startpage.com which I have never heard of, but turned out to be its default; application state had been lost. This was after a spell of getting my login password wrong due to being half-asleep whilst waking up and forcing a power cycle. I had to repoint the Default Search Engine from within Settings->Search->Default Search Engine; I have no idea how this could have regressed.
@bms48 Startpage, formerly IxQuick, is as far as I know, still a Dutch search engine, with focus on privacy. Maybe the default in Vivaldy?