Sometimes, after upgrade BoostSolutions product, user cannot get access into sites. The error message could be "SharePoint Foundation" or "File not found".
This issue is caused by the conflict between the old version "sharepointboost" and the new version "boostsolutions" while the old version "sharepointboost" hasn’t been removed completely during the upgrade.
To solve this issue, please follow below steps:
1. Go to Central Administration—>System Settings—>Farm Management—>Manage Farm Solutions to check whether there are wsp with the name "sharepointboost". If so, retract and remove it.
2. Reinstall the upgrade again, choose "Repair" during re-installation.
Applies for: SharePoint 2013, SharePoint 2010, SharePoint 2007