Sign In Required for users with Acrobat Pro 2017 after a year

We've deployed Acrobat Pro 2017 to our office systems using an .mst file created in the Acrobat Customization Tool. We added our license key to it so our users would not get the Sign In Required prompt.

Over the past week, we've had some users get the prompt when using Acrobat. We're wondering if there's a web-connection issue that's preventing the client from phoning home to an Adobe web site and maybe the prompt is due to that. Does Acrobat check in to a web site to verify the license key at each start?

I've been looking through some logs that I think might help troubleshoot the issue, amt3.log, oobelib.log and PDApp.log. There are some errors in there but frankly I'm not sure what I'm supposed to be looking for.

Our office, in their infinite wisdom, neglected to purchase support when buying Acrobat so we do fly blind sometimes trying to sort things out. Has anyone run into a similar issue and found an answer to the mysterious Sign In Required prompt?