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.