On several of my client’s sites they are no longer to add a document to Memphis Documents Library due to a conflict with Modern Events Calendar plugin. To upload any documents, you have to disable MEC and then upload and then re-activate MEC.
I’m wonder if it’s to due Version 5.13.1 being used and unable to update from ThemeCo’s to Version 5.13.5, Not sure if that would fix, but worth a try.