postgresql – relation does not exist

After upgrading from postgres 9.x to 11.5 my Postgres queries started giving lot of errors in the pg_logs.
It is running fine and suddenly with no change on the server, it start giving errors for some time and then disappear (self fix) and the remaining session continue smoothly.
The first error in the pg_log is following and it comes for all 8-10 tables in the database :
ERROR: relation “versions” does not exist at character 518
To fix We have performed :-

  1. “vacuum full verbose analyze”
  2. schema check for tables and queries
  3. permission for tables to the users.

But still, the issue persists. can anyone help where to check for root cause.