URI Protocol Handler in Internet Explorer stops working

You built a protocol handler, then, after the September 2021 updates, it stops working. Here's a possible answer as to why.

In September 2021, one of the Microsoft Windows security updates did something to the requirements of URI protocol handlers. I do not know which one. The handler names used to be pretty free-form without any restrictions. However, that is not the case now. I found 2 restrictions to the name that broke our handler.

1. It must start with a letter and not a number
2. It must be lower case

Example of what used to work (not the actual value): 123SUPERSECRET456

The same thing had to be rewritten as something like: supersecret123456

As soon as we figured that out through a bunch of trial and error, we got our handler working again. We still have a lot of work to do to our sites that use the old handler, but we now have a solution.