OK, at quick glance, it's the same nature of the problem that it used to be when it broke last time (identifying whether the browser is Vivaldi (and not Chrom)). However, this time (so far) there's no known solution to it.
(Note folks complaining about vivExtData no longer working in 6.7):
https://stackoverflow.com/questions/68659729/how-to-detect-clients-web-browser-is-vivaldi
Given that it's a recent issue, people will probably discover a workaround sooner or later.
Meanwhile, we could just hard-code the check to always treat the browser as Vivaldi (I checked it and at quick glance it works), but that would break it for Chrome (so I can't imagine this change accepted upstream).
You can find the fork (based on the most recent upstream) with the workaround (hardcoded check for Vivaldi) here: https://github.com/grigorye/chrome-otto-tabs/tree/feature/Vivaldi-Only