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!
@piotrbarczuk Password is your general Opera account password. Passphrase is the password you have chosen to use with your synced data so it is totally encrypted.
For obvious security reasons, the only way to get a new passphrase is by reseting your synced data, what means that all data will be removed from the sync servers.
@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.