Awkward dialog box: MZFinance.InAppBuyLoginRequired_message
Very similar error messages have been noticed by developers and users alike.
Check out this blog post. It's not an identical error message but perhaps this will fix it for you as well:

AppleCare found the solution and it is rather simple. Now this solution has worked for me, but there is no guaranty that it will work for you.
Solution: Go to Macintosh HD (or whatever you call your HD in Finder) &gt; Library &gt; Preferences &gt; SystemConfiguration
For there look for the file labeled: “NetworkInterfaces.plist”

So, you might test this remedy by making a good backup of your Mac and ensure you either have the network passwords and VPN setup files you need (or can regenerate them / reset your WiFi routers to passwords you know) and delete the NetworkInterfaces.plist file.
If that fails to work, you may need to open a Mac App store support ticket and involve Apple in troubleshooting this further. If you can document the steps needed to reproduce this error, please post a follow up question.
Chances are there is a bug in some Mac App Store software that was downloaded onto your computer and getting Apple support involved mit fix it for you and others at the same time.