Found a solution suggested to someone with a different error accessing the control panel after updating to version 4.3.
Tried it and it worked, so writing it here in case anyone else needs it…
The problem was caused due to one of the plugins that was incompatible with new version. Just deactivate manually all plugins through phpMyAdmin (https://codex.www.ads-software.com/FAQ_Troubleshooting#How_to_deactivate_all_plugins_when_not_able_to_access_the_administrative_menus.3F). If it solves the problem (which it did for me), go to your WordPress control panel and start reactivating the plugins, one at a time to see which one causes the problem.
Good luck ??
