Good morning @asmecher,
Your instruction in other post resolved the issue in 2 steps: Cannot use a scalar as an array - #21 by asmecher
You can safely delete all metaCitations rows from OJS 3.x – they’re a hold-over from OJS 2.x that is no longer useful. Execute:
DELETE FROM journal_settings WHERE setting_name='metaCitations`;
Then remove all .php files from your cache/ subdirectory.
root@ojs-upgrade-2:/var/www/html/ojs-3.1.1-4/cache# rm *.php
Best regards,
Dung.