Like some other posts in this thread, I have found that WebPlus (X7 in my case) in Windows 11 will not open ".wpp" files if I double click them in the file explorer. The app starts up ok, but the file is not opened in the app.
I have discovered that this is due to the shell open key using the dde method to pass the fileneame to the app.
I fixed this using "regedit", but be careful with this tool, "here be dragons" and all that!
1. Look for the key "HKCR\WebPlus.Document\shell\open\command"
2. In that key change the "(Default)" entry to be "C:\Program Files\Serif\WebPlus\X7\Program\WebPlus.exe" "%1"
3. Beside the "command" key is one called "ddeexec". Rename it to "ddeexec.old".
4. Now we have to change some permissions for the "command" key, or WebPlus restores the dde stuff...
5. Rt click it, select permissions, press advanced, press disable inheritance and convert it to explicit permissions.
6. Ok back to the dialog where you can change the permission for your username. Remove your own "Full Control" for the key, leaving just "Read".
7. Click ok, and the job is done.
Similarly for PagePlus (X7, in my case again) and ".ppp" files...
"HKCR\PagePlus.Document\shell\open\command"
=> "(Default)" = "C:\Program Files\Serif\PagePlus\X7\Program\PagePlus.exe" "%1"
...and the other renames and permissions tweaks.
Obviously, you must amend the details if you have a different version, X5, 6, 8 or whatever.
Enjoy, Ian