What is relation 266775 of database 196511? Is it cms_c207c1e2_0ce7_422c_aafb_77d43f61e563.cms_item or some system catalog table?
When I search google for "ExclusiveLock on extension of relation" I find one thread about shared_buffers being very high but not big enough to fit the entire data in the cluster. How much ram, what is shared buffers and what is the total size of the database(s) on that Postgres instance?