5.1 is a much older version and lots of issues have been resolved since then so this could have been a bug back then.
The basic steps to perform an upgrade if you were upgrading to v5.6: uninstall 5.1, clear your browser cache, install version 5.6 on any of the web-front end servers, upload a valid QWP license file using "Manage Web Parts License" feature in SP Central Admin, and launch Configuration Editor tool to fix any errors in the web.config file.
You can find details of upgrade instructions as well as list of resolved issues in our Release Notes if you google 'Quest Web Parts for SharePoint 5.6 Release Notes'.