yes thanks, it was the plugins error. I had to delete all the plugins then it worked. But now when I reinstalled the LDAP plugin, it does not look to work fine. firstly, for agent account, I have to manually add for LDAP authentication however, it was already preconfigured on previous version before migration. Also the already registered users are not able to login from ldap. Once they enter the password it again shows blank screen. In logs it is showing "Invalid CSRF Token".
