I figured it out, it has to do with the version of Java I had installed. So if anyone else is encountering this problem with Ubuntu 18.04, try installing a different Java.

@Philfou The passphrase isn't stored anywhere but your computer, that's why there can't be a recover method for it, Opera simply doesn't know it.

SOB... finally got the passphrase I used and sync is working on my new PC.... I was almost 99% I have tried it before, but 100% not, since now it works I went systematic over all the possibilities and finally "find" right "word". Thanks to all who tried to help me!

Ok, thank you very much @leocg, so I'm going to do this.

@leocg Thanks! Seems like this did the trick. I expected that there might be something like that in settings but for some reason I find it quite hard to find a thing I'm looking for in there every time I try... Took me a few minutes to find this as well but at least I knew that it should be somewhere so I didn't give up halfway through haha.