I used to be able to highlight the files I wanted to join, right click and choose "PDF24" and it would bring up the PDF24 File tool where I could choose "Join" among other options.
Now when I right click and choose "PDF24" it asks how I would like to open the files.
I have tried uninstalling and reinstalling PDF24, no resolution.
Re-install and enable the new shell extension DLL. Then right-click the files you want to merge and select pdf24->combine/merge. If you do not select the new shell extension dll, then there is no cascaded menu in the context menu, but the old variant which should open the PDF24 files UI.
Thank you so much, that fixed it! Had to do a manual install instead installing through the company software center to be able to select "shell extension DLL" but that did the trick!