HDCP error on DELL U2413 when using iTunes playback

My U2413 has suddenly stopped playing back iTunes content. The error says that this display doesn't support HDCP protection. I was using a display port to thunderbolt connection and playing content on a 2011 mac mini.

This computer connects and plays video content on my panasonic plasma television via HDMI cable. When I switched to HDMI with this monitor I get the original message. However:

a) The U2413 worked fine last week playing i tunes content with the display port connection

b) The U2413 works fine when connected to my macbook pro with the display port connection

It seems to me more like an Apple software (iTunes issue) as the connections all worked fine last week. The Macbook is running Yosemite and the last but one version of iTunes. The Mac Mini is running El Capitan and the latest version of iTunes.

I spent about 8 hours yesterday with apple support and they say the issue is with the monitor. They tried everything else.

Can anyone help me?