Hope this may save someone some time and perhaps some hair that they might otherwise have forcibly removed 8-).
Was experimenting with a new 7.2.1 installation yesterday, logged on as host, and came across the list of Available Modules under Admin | Site Settings | Advanced Settings | Host Settings | Available Modules.
Noticed that there were two modules unchecked that might be useful, DNN_iFrame and DNN_Media. So I checked the boxes next to those modules, then clicked the Update button. Then went on to other tasks.
Eventually I clicked on the Host | Extensions item and got a page with the message "DNN Error" and a button labeled "Return to site." Could not get the Extensions tab to load.
The log showed that the error was
DotNetNuke.Services.Exceptions.PageLoadException: Cannot use a leading .. to exit above the top directory.
Research suggested that this error arose from a leading ../ in a relative path somewhere. Searched the entire database looking for the string '../' and found it in 2 locations, each in the IconFile column of the dbo_Packages table. Sure enough, one was supposed to be the path for the DNN_iFrame icon file, and one the DNN_Media icon file.
Ran the following script in SSMS to fix the DNN_Media icon file path:
IconFile = '~/images/icon-media-32px.png'
Name = 'DNN_Media'
and this one to fix the iFrame icon file path:
IconFile = '~/images/icon-iframe-32px.png'
Name = 'DNN_iFrame'
After the update the Extensions tab loaded once again.
Hope this helps someone, and all the best