I have same problem

go to "www.sync.opera.com"

send me to choose account

choose account or login are same send me to

"auth.opera.com/account/edit-profile"

click "See your synchronized data" send me to

"auth.opera.com/account/redirect?return_url=...." this page shown a few second and redirect to

"www.sync.opera.com" and got "HTTP ERROR 500"
i got this loop 1-7 (i try loguot and login again got same result)

but go to opera forum i got 2.-3. and send me to forum no error no problem