I have the same problems since v6.7.x @ work. Although my Win11-workstation is behind a hardware firewall with an exclusion for "bifrost.vivaldi.com", sync worked. Not flawlessly, because often there were network connection errors, mostly with upload, but at times sync was successful. Last version of Vivaldi when sync was possible was the latest v6.6.x.
Now, with v6.7.x it hangs at "Initialising Sync...", when you disconnect and try to connect again. The strange thing is, when I uninstall v6.7, install v6.6 it works immediately and sync is successful. Then I update to newest v6.7.x and sync works for some time and then stops again because of network errors.
They must have changed something with the Sync-engine, but not for better... 😕 With all other browsers (Firefox, Chrome, Edge) syncing accounts works flawlessly. So why not "copy" their technics to make sync in Vivaldi work better?! 🤔
And here is what "tracert" says: sometimes a request timeout occurs, but not always. 😉
PS C:\> tracert bifrost.vivaldi.com
Routenverfolgung zu bifrost.vivaldi.com [31.209.137.10]
über maximal 30 Hops:
1 <1 ms <1 ms <1 ms 192.168.***.***
2 <1 ms <1 ms <1 ms 83.***.***.***
3 1 ms 1 ms <1 ms 83.***.***.***
4 <1 ms 1 ms <1 ms 83.***.***.***
5 * * * Zeitüberschreitung der Anforderung. <----
6 6 ms 6 ms 6 ms ffm-bb2-link.ip.twelve99.net [62.115.138.46]
7 15 ms 15 ms 15 ms prs-bb2-link.ip.twelve99.net [62.115.122.138]
8 20 ms 21 ms 21 ms ldn-bb2-link.ip.twelve99.net [62.115.133.238]
9 30 ms 30 ms 30 ms dln-b4-link.ip.twelve99.net [62.115.120.105]
10 29 ms 29 ms 29 ms milaehf-ic-377054.ip.twelve99-cust.net [62.115.148.75]
11 55 ms 55 ms 54 ms 157.157.69.221
12 55 ms 55 ms 55 ms 206-99-22-46.business.hringdu.is [46.22.99.206]
13 56 ms 54 ms 54 ms 31.209.137.10
Ablaufverfolgung beendet.
PS C:\> tracert bifrost.vivaldi.com
Routenverfolgung zu bifrost.vivaldi.com [31.209.137.10]
über maximal 30 Hops:
1 <1 ms <1 ms <1 ms 192.168.***.***
2 <1 ms <1 ms <1 ms 83.***.***.***
3 1 ms 1 ms 1 ms 83.***.***.***
4 1 ms <1 ms 1 ms 83.***.***.***
5 1 ms * * zch-b1-link.ip.twelve99.net [62.115.41.128] <----
6 6 ms 6 ms 6 ms ffm-bb2-link.ip.twelve99.net [62.115.138.46]
7 15 ms 15 ms 15 ms prs-bb2-link.ip.twelve99.net [62.115.122.138]
8 21 ms 22 ms 21 ms ldn-bb2-link.ip.twelve99.net [62.115.133.238]
9 30 ms 30 ms 30 ms dln-b4-link.ip.twelve99.net [62.115.120.105]
10 29 ms 29 ms 29 ms milaehf-ic-377054.ip.twelve99-cust.net [62.115.148.75]
11 55 ms 55 ms 55 ms 157.157.69.221
12 55 ms 55 ms 55 ms 206-99-22-46.business.hringdu.is [46.22.99.206]
13 54 ms 54 ms 54 ms 31.209.137.10
Ablaufverfolgung beendet.