@gaelle I wanted to give y'all an update. I upgraded to the new build of Android Vivaldi released today (219) but still having issues with sync. Currently my settings are that I only have Bookmarks set to sync on Android, all the other sync options are disabled. It is displaying green checks next to both upload & download with a timestamp of about 2 hours ago. When I first upgraded after about 2 hours I checked and it was reporting the same Network error described in the bug, and the retry timer ran past 0 into the negative. After a little bit of watching the timer continue to go negative without apparently retrying, I changed the settings from Synchronize All to just syncing the bookmarks and applied the new settings.
As long as no error is displayed when I check, I'm going to let it be for at least another 2 hours. I think that they're are at least 2 issues underlying this bug. There are the errors of course, but also displaying success when the data hasn't actually been downloaded and applied. As mentioned in the report, the app reported success nearly immediately, but only about 4 hours later did my bookmarks and other content become available in the app.
Hopefully that's what is happening right now: incorrectly showing success while still processing and Vivaldi Android will eventually complete the sync. If I get the time, I'll try sniffing the network to look for the sync traffic, but I can't promise I'll get to that today.