Recently we had migrated one InfoPath form from one environment to another environment in SharePoint online site. The form was published to an SharePoint online form library. Now when go to the library settings and then click on “Advanced Settings” which is under General Settings. Then click on “Edit Template” which is under “Document Template” to open the form in InfoPath designer 2013 like below:
But instead of opening in the InfoPath 2013 designer, it gave the below error:
There has been an error while loading the form.
Click Try again to attempt to load the form again. If this error persists, contact the support team for the web site.
Click Close to exit this message.
And in the details error message the error was like below:
The URL FormLibraryName/Forms/template.xsn is an unsupported relative URL. To fix this problem, use a site-relative URL starting with ‘~site’ or a site collection-relative URL starting with ‘~sitecollection’. You can also use an absolute URL. See the fig below:
We figure out that the problem is happening if Modern experience has been enabled for the form library. So we changed to the classic mode of the library. And then we were able to open the form using InfoPath designer 2013.
Hope this will help.