After T137973 https://commons.wikimedia.org/wiki/Special:CentralAuth/Biplab_Anand has a number of unattached accounts. He is unable to attach them.
I asked @biplabanand to verbosely log a request with X-Wikimedia-Debug set to mw1017.
It successfully was able to repro with a Login unification not complete! message, so we have logs of what happens: Kibana dashboard of biplabanand test request
No exception related to CentralAuth is thrown.
Maybe you can paste the error here? Most people don't have access to Kibana, if it's not the kibana for beta.
As far as I can interpret the logs, there isn't any message at error level related to CentralAuth.