Either there is no default mail client or the current mail client office 365

I hated this error sign recently, couldn't find a way to solve it. Just now I managed to find a solution. Open Regedit to HKEY_CLASSES_ROOT\Installer\Components\F129....(something something).

Now right click on that folder and choose Permission. Add Everyone, Allow a permission to Read.
That's it. You don't even need to restart the PC.