Galaxy Tab s2 can't install apps from SD card now. "Application not installed."

A

AC Question

I'm using a Galaxy Tab S2 and I never had a problem installing apps from the SD card so far.
Now when I go to install an app from there it says "Application not installed."
I am not rooted so I do not have access to some of my files.
I am not ready to root it because I have no faith in myself to not brick it.
I have heard it can be caused by trying to install older versions of an app over the newer version that was already installed.
Installing works when I move the app to internal memory. (I have not tried this with an app that is an older version than I have installed).
I have installed from unknown sources enabled.
I am using solid explorer for navigation and installation.
What is my best bet on how to fix this?
I would rather deal with the glitch than reset the tablet because I have spent a lot of time setting it up.
I guess I could take the files from the SD card off of it - mostly media, format it, and put the files back. I have a feeling it wouldn't help.
Thanks for listening.
klear6