I haven’t been through but one WAS (Web Application Server) update to the new 101d patch of v26, but I wanted to get this info out ASAP for anyone else that may run into it.
I suspect this may be an issue that only effects Apache installation, which is what all of our productions servers are. And like I said I have only done one update so far so it might not even effect all of them.
After updating the WAS to 101d on the first server we upgraded, when we launched the website instead of our normal website we got this.

Now understand that we always create a index.html file that redirects to the url with the project shortcut, so that our users only have to enter the domain. For example they enter https://ctls.labdonors.co.uk and get redirected to https://ctls.labdonors.co.uk/LabDonors.
In Apache this depends on our <VirtualHost> entry having a Directory Index similar to this
DirectoryIndex index.awp index.htm index.html
After updating to 101d the DirectoryIndex line was changed to
DirectoryIndex .WDApplications.awp index.awp index.htm index.html
And it was the .WDApplications.awp that was causing that other page to launch. I moved it to the end of the line, or you could remove it completely if you want
DirectoryIndex index.awp index.htm index.html .WDApplications.awp
And once I restarted Apache our site now launches as expected once again.
Again this information is very preliminary and I will update it as I learn more but wanted to get it out there as soon as possible, and as you see the fix is very easy if you do have the issue