This problem seems to happen whenever changes are made to your browser, or sometimes for no apparent reason.
It can include:
If it was working a day or two ago try restoring the computer to a recent Restore Point. This can be an easy fix and it's worth trying first.
In most cases the fix is easy.
If desired Firefox can be restored as the default browser.
Even though Windows 10 comes with the latest "Edge" browser Internet Explorer is still needed for older Microsoft Office suites it seems. I have 2007 installed and thinking I no longer needed the old Internet Explorer 11 I removed it, and the problem returned.
So Internet Explorer 11 was reinstalled and Outlook can once again open links.
This is not an Outlook error. The most common cause is a corrupt registry key in Internet Explorer caused by installing or reinstalling Chrome or Firefox.
Before starting it would be wise to create a Restore Point first. Hit the Windows key and type in "restore point" and Windows 10 will show a link going directly to it.
The registry key is found at:
... and is either damaged or corrupt.
If possible export a key from a working computer and import it into the problematic machine. Otherwise try editing the registry as below.
Next make sure that the registry entry string default key of
HKEY_CLASSES_ROOT \.html contains the string
htmlfile as shown below. Repeat this step for
If it only fails for your user edit the keys: