I have a 2015 Macbook Pro connected to my recent Dell P2715Q monitor with DisplayPort connection. The monitor displays well during normal operation but when I sleep my notebook and wake it later, I come back and my notebook is frozen. After several calls and a visit to the Apple Store they were unable to find an issue with my computer. We tested with a Apple Display in their store and there was no issue. Is there a firmware update for the monitor? I'm fairly certain the issue is the Dell causing my notebook to freeze. Help! So frustrating!
Unfortunately, as you can see in the image below taken directly from the P2715Q page, the Apple proprietary operating system and hardware were not tested/validated by Dell.
The Apple store should have tested your 2015 Macbook Pro on a Dell monitor to try to replicate the issue. We have no idea as to what in the Apple operating system is failing when coming out of sleep mode. You should post on the Apple Support Communities and see if they can offer some insight or troubleshooting steps.
By disabling DDC/DI in the monitor OSD (On Screen Display) -Others. Did that fix the issues?