I had a similar problem but the root cause was that my app was pointing to the old dev database which had exceeded it’s limit of 10,000 rows.
Although I created a new Basic db and backed everything up, the app was still pointing the old dev DB.
heroku pg:info
Check to see the rows: 10300/10000 (then you have a problem)
You will need to
1) Create new DB with more rows (Basic or the “Production” ones -> Heroku seems to be forcing an upgrade to make more money errrrrr)
2) backup the old DB using pgbackups: heroku pg:backups:capture SMALL_DB_NAME
3) restore the backup to the new DB: heroku pg:backups:restore BACKUP_ID BIG_DB_NAME (see links below for more details)
4) PROMOTE the new DB to the primary for the app: heroku pg:promote BIG_DB_NAME
can always utilize:
heroku maintenance:on(to disable the app while updating)
heroku maintenance:off
heroku pg:info(to check the status)
If this is the problem you may want to check out:
https://devcenter.heroku.com/articles/heroku-postgres-starter-tier
https://devcenter.heroku.com/articles/migrating-from-shared-database-to-heroku-postgres