@[email protected] I need to apologize, the issue was on my end.

Since this was unknown so far, I wanted to test a bit, before filing a bug report.
Long story short: Eventually, after reseting my sync data, and my mobile and desktop Vivaldis, I made a fresh desktop profile (imported bookmarks etc another browser), synced it. And everything went smooth.
There was something wrong with my desktop profile, which caused further trouble with the mobile version.

Thanks for pushing me in the right direction <3