The following module gives a (possible) response to the questions related to "TransactionRollbackError:could not serialize access due to concurrent update CONTEXT: SQL statement "SELECT 1 FROM ONLY "public"."res_users" x WHERE "id" OPERATOR(pg_catalog.=) $1 FOR SHARE OF x": https://bitbucket.org/xcg/base_res_users_streamline/overview
Read carefully the Description section in the manifest file: ATTENTION though: this module will render the "last login" column on your users definition useless since it disables the SQL updates associated to the user login phase to obtain better performances.
After installing this module on Odoo v8.0.1 I got an error 500 when trying to login.
What module are you talking about?
Maybe you will have to uninstall that module or restore a previous backup of your db...