• Go to https://auth.opera.com/ click Sign out
  and Sign in
• restart opera browser
  everytime i got this problem (frequently)
  it's work for me.
  i don't know what cause of problem.

I have same problem

1. go to "www.sync.opera.com"

2. send me to choose account

3. choose account or login are same send me to

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

5. click "See your synchronized data" send me to

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

7. "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