Just had a stressful few hours but error is now resolved...
Had this error:
An operation failed because the following certificate has validation errors:\n\nSubject Name: The root of the certificate chain is not a trusted root authority..
Basically anywhere where you try and open a infopath form you got an data source error saying access was denied and you must look on the Application log.
The application log was full of the errors above. Also in the ULS Logs.
Everywhere i searched pointed me to Trusted Root Authorities and "Manage Trust" under SharePoint Central Admin...Nothing worked...
Then this error popped up once:
Insufficient SQL database permissions for user 'Name: domain\Username SID: S ImpersonationLevel: None' in database '' on SQL Server instance. Additional error information from SQL Server is included below.
The EXECUTE permission was denied on the object, database, schema 'dbo'.
The user in the error was the SharePoint installer account which was suspicious. The error are in the logs for other users.
We compared the security rights in SQL for the user to QA and it only had Public access rights.
Added SecurityAdmin and DBCreator and after a IISReset is was working again.
Got no idea how the rights might have changed but at least it is solved now.
Hope this helps you.