7-zip cannot open file on double click

The problem is that the default is opening with 7zG (gui) while it should open with 7zFM (file manager).

You can change this using a program like DefaultProgramsEditor

Good luck.

You could try to reset the associations from within 7-zip File Manager. One thing to note, if you are using Windows 7, you need to run 7zFM as administrator so it can set the changes.

Some registry keys were pointing to an x86 version of 7-Zip, which for some reason can't handle being invoked directly on this machine. I am not sure whether this is a bug in the x64 version of 7-Zip or whether a x86 version was installed on this machine before and didn't uninstall cleanly.

The following registry keys referred to the Program Files (x86) directory. Changing those to refer to the Program Files directory fixed the problem.

HKEY_CURRENT_USER\Software\Classes\7z_auto_file\shell\open\command
HKEY_CURRENT_USER\Software\Classes\Applications\7z.exe\shell\open\command
HKEY_CURRENT_USER\Software\Classes\Applications\7zFM.exe\shell\open\command

As mentioned in a comment by @RolfBly, this can also be resolved as follows (Windows 10):

1. Right-click on the file
2. "Open With..." or "Open With..." > "Choose another app"
3. Tick the "Always use this app to open .7z files" checkbox at the bottom of the popup
4. Click "More Apps"
5. Scroll to the bottom of the list, click "Look for another app on this PC"
6. Navigate to the 7zFM.exe application - typically in Program Files/7-Zip
7. Select the 7zFM.exe application, OK

.7z files will now open in 7Zip as expected