InfoPath 2010 browser-enabled forms cannot be opened - "The following document is not a valid InfoPath form" - SharePoint Rita - Site Home

Environment:

SharePoint 2010, InfoPath 2010 browser-enabled forms

Issue description:

We have alot of InfoPath forms in SharePoint 2010. These forms were migrated from InfoPath 2003 client to InfoPath 2010 browser-enabled. Occasionally a few InfoPath forms cannot be opened in browser with the following error:

There has been an error while loading the form.

The following document is not a valid InfoPath form: {path of form}

Cause:

After doing some research and comparision, I noticed that we have a rich text field in the form that users use to copy and past html content from the internet. The pasted content includes the following string that caused this issue:

<?XML:NAMESPACE PREFIX = "[default]" http:="http:" xmlns:http="urn:http" ?>

Resolution:

Download the form to local machine. Open the form in notepad. Find the above string and delete.
From SharePoint library, find the affected item.
Checkout the item (if checkout is required)
Upload a file to the library, point the path to the local path of the newly saved xml file.
Save
Open the form from SharePoint, it should open in browser without any issue.