Hi, sorry for taking too long to reply. Thanks for your advice!
I tried your methods - well, first I uninstalled via Windows 10 uninstaller then reinstalled EN - this didn't correct the problem. I then used a free, open-source uninstaller program then reinstalled EN - this didn't work either. I then used the program you recommended (Revo Uninstall), then reinstalled EN - and this didn't work either!!!
However, I think I know what the problem is. This issue occurs on my Windows 10 laptop with an internal 512GB SSD. A few weeks ago, I got Windows 10 notification about my SSD filling up and that I could use Windows 10 to manage my storage. Essentially Win10 offloads stuff from the SSD to the cloud (I'm guessing my OneDrive account). I decided to "accept" that proposition.
So now I have a lot more space on my SSD, but I think Win10 moved the EN database to the Microsoft cloud. This might explain why EN can download something (download from my OneDrive cloud account to the laptop), but can't immediately open a file (it first has to download the file from OneDrive, then open the file, which is too much to handle, I guess).
I think that this is the problem, but I'm not going to go through the steps of putting everything back on my SSD to find out. But if anybody else is reading this and experiencing the same problem, consider this as a possibility.