Double Click Won't Open Files Unless I Quit the App

Ever since installing High Sierra, I cannot double click files to open them if the app is already open.

For example, if I am trying to open a Word doc, I can double click it and it will launch the app and open the document. But then if I need to open a second document, I double click and either nothing happens, or occasionally I will get a dialog box that says "Microsoft Word is no longer open." Either way, the second file will not open unless I open it through the app (File > open > etc.) or quit the program.

This has been happening with ALL of my apps and file types, including web links. If I try to click on a link in my email client and my browser is already open, I get the same error or sometimes it will open a blank window, but clicking on a link will launch a closed browser. I have to ctrl + click to copy the link and paste it into the browser.

I have a high volume of files and links I have to open simultaneously for work, and it's very frustrating and time consuming to have to manually open each one within the app instead of double clicking!