I posted this earlier but it has disappeared (?). It is not exactly a coding question but probably more a server question.
I created an up which uploads two csv files as data frames and then uses information from one to create an addtional column in the other. The resulting modified data frame can then be downloaded as a csv file. The app worked fine locally (mac book) but when I moved it to my shiny server (Ubuntu 16.04.5; Digital Ocean) it would upload and display the two csv's correctly but would immediately disconnect from the server when I clicked on the action button to modify the one using the other. I have never had this problem before and I have at least three dozen shiny apps running on the server (all of which continued to work fine).
I removed the app, made a few modifications unrelated to the problem (minor edits, cosmetic changes), installed system updates on the server, redeployed the app and, et viola! , it ran fine. I have seen a few other posts about similar issues but none clarifying what the ultimate source of the issue was. Gremlins? I'd like to know how to avoid this issue in the future. Any thoughts would be appreciated.