I don't know very much about this, but we are starting to switch from Access to SQL Express and SQL Standard on our app, and I've actually seen this message in the last couple of days.  We see it when I try to access the SQL Express that is running on a different machine.  To do that, there is a port that must be opend up in SQL Express.

I doubt that you are trying to hit your Apex database from a different computer than where it was installed.  But I wonder if you have 1) renamed your computer recently, or 2) if you might be logged on with a different ID than you had when you did the original install?

Those are just brainstorming guesses.   I would be interested in hearing what Stellar Support tells you.