I recently pushed an upgrade from moodle 3.4 to 3.7 to my sandbox site running on a windows box, everything was running fine before the upgrade.
This involved upgrading the PHP from 5.5 to 7.3.8 and MySQL from 5.1 to 5.6.45 (so that it would all be compatible)
Upgrade seemed to work fine except the Global Search function, I'm getting the error "Engine solr is not installed", so obviously it means either the Solr instance isn't running or not installed properly.
But the Solr instance seems to be running fine (and was also running fine before the upgrade). I have my production site on the same server, using the same Solr Instance that works too. I can see both Solr Cores (production and sandbox) on the Solr ui and all the settings are fine.
To help diagnose the issue, I turned off Solr and used Simple Search, this changes the status on the manage global search page from an error to a green label (saying "Simple Search"), but instead I get an error saying "Error reading from database"
Has anyone encountered this before?