I too had the problem of password not being accepted while trying to sync in "Opera browser: Fast and Secure"
I tried your procedure of resetting the sync data from the server. Now when I try to resync Opera on my tablet it immediately crashes with an abrupt close down. This then requires a soft reset to bring the tablet back to life. Not something I intend repeating very often. Twice is enough.
I am using a Hudl 2 tablet running Android 5.1, with Opera version 48.2.2331.133274
I would like to stress that in all other respects Opera works very well and remains my favourite browser.
I hope this serious but can be resolved soon, because the sync capability was a major plus point.