1. Did you use any Custom Code in Child Theme? Please try to remove it and check back again.
2. Did you make changes in DB?
3. Was the prefix of tables changed?
4. Is the error there if all 3rd party plugins are disabled? Please try this as well.
This seems to be the server-end database issue, so please once reach out to your hosting provider as well, they will check it for you as well.
Regards 8Themes Team.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.