This article describes an issue experienced in the following bug:
MultiSites: If user creates form via Forms gadget from site2, gets redirected to site1
Upgrading to the latest version of Episerver Forms did not resolved the issue.
In this case the the Episerver Forms folder is still in For This Site location not in For All Sites location. The workaround is to move all Forms from Episerver Forms folder in the Asset pane to a temporary folder and then delete that Episerver Forms folder. After restarting your site, a new Episerver Forms folder will be created automatically in the "For All Sites" location and they can move all Forms back to that folder.