These question is for both of you...
Are you running your system with wireless or ethernet?
What type of connection do you have to your ISP? (T circuit, DSL, Dialup, Cable, Fios)
Based on both of your issues, it is your router asking for the credentials, and has nothing to do with MediaSignage. You system is going out to their site and your router is saying it needs authentication to access the internet to get to that site. If you are running DSL, these typically require a username and password for your ISP to access the internet. There have been issues with Viruses/Malware mimicking the same issue. Please run a scan on your systems. I highly recommend using Malwarebytes as an additional tool and should be run in Safe Mode.
This is the only thing I can think of as to why your router would be asking for credentials. If it was not your router, I would suggest adding medissignage to the Trusted Sites area in IE, if that is what you are using. I guess it couldn't hurt anyway.