I know this waaaaay late, but I just ran into this problem, and I believe that the problem is that you exceeded the number of rows allowed for your database. In my case, I used the Hobby-dev plan, and the limit is 10 thousand lines. If you run heroku pg:info , you will see something like:
If Write access revoked is a key piece of information. Upgrading your database (or deleting some data) should solve this problem.
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