So i logged-in for syncing from my pc-opera again. And here it looks different.
When i open "tabs" it shows my open tabs on my pc and has two sections for my tablet.
One section is listing those 5 tabs, which are actually (really) open on my tablet. The second section (names exactly the same) shows the open tabs before i reinstalled opera on the tablet.
In https://www.sync.opera.com/tabs i find the same sections, but those 2 sections for the tablet show exactly the same tabs, and it is as an addition from what i see now here on my pc (new install and former install mixed together and doubled)
There is definitely something going wrong.