Obrigado, Gerhard! Agradeço sua ajuda. Essa situação está ficando cada vez mais estranha.
Também não faz sentido para mim que houvesse um problema com incoming_referers. Fiz uma restauração da produção para este site de staging tão recentemente quanto em outubro.
Parece que há algo errado com o banco de dados ou com os índices. Eu suspeitava de um plugin, mas os erros ocorrem em índices tão diversos que parece improvável que um único plugin seja o culpado. Um grep anterior nos plugins em busca dos problemas de índice quebrado não retornou nada.
Em uma restauração, obtive o seguinte:
[2020-01-03 16:25:30] ERROR: could not create unique index "index_plugin_store_rows_on_plugin_name_and_key"
Fiz:
[10] pry(main)> PluginStoreRow.where(plugin_name: 'discourse-data-explorer',key: 'q:-8').destroy_all
E tentei novamente:
[2020-01-03 16:49:16] ERROR: could not create unique index "index_tags_on_lower_name"
[2020-01-03 16:49:16] DETAIL: Key (lower(name::text))=(addins) is duplicated.
Aqui está o cabeçalho completo da restauração mais recente:
[2020-01-03 16:41:34] 'pfaffman' has started the restore!
[2020-01-03 16:41:34] Marking restore as running...
[2020-01-03 16:41:34] Making sure /var/www/discourse/tmp/restores/default/2020-01-03-164133 exists...
[2020-01-03 16:41:34] Downloading archive to tmp directory...
[2020-01-03 16:45:20] No metadata file to extract.
[2020-01-03 16:45:20] Validating metadata...
[2020-01-03 16:45:20] Current version: 20191220134101
[2020-01-03 16:45:20] Restored version: 20191129144706
[2020-01-03 16:45:20] Extracting dump file...
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:15] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] CREATE INDEX
[2020-01-03 16:49:16] ERROR: could not create unique index "index_tags_on_lower_name"
[2020-01-03 16:49:16] DETAIL: Key (lower(name::text))=(addins) is duplicated.
[2020-01-03 16:49:16] ERROR: current transaction is aborted, commands ignored until end of transaction block
[2020-01-03 16:49:16] ERROR: current transaction is aborted, commands ignored until end of transaction block
[2020-01-03 16:49:16] ERROR: current transaction is aborted, commands ignored until end of transaction block