The HD5xxx series has reached peak optimisation and been moved to legacy status, there will not be any further driver updates for your product. More information available here.
The latest Crimson driver for your system is 16.2.1 and is available here.
The last Catalyst driver for your system is 15.11.1 Beta and is available here.
I would recommend using DDU to sweep your system of AMD drivers, and then trying one or both of the drivers above to see if they resolve your issue.
Thanks for your reply Matt. Alas, it does not work....
Cleaned system with DDU and downloaded both recommended Crimson and Catalyst drivers. Tried to install Catalyst 15.11.1. After Finish nothing happens, no reboot. Manually rebooting does not help. Windows still shows general PnP driver with just the one monitor.
Run DDU again and tried the same with Crimson 16.2.1, same result.
This was also the problem with Catalyst 15.7.1: it seems to install, but Windows does not recognize the driver as being installed, only the AMD Installation Manager program (in Configuration).
Is the system supposed to reboot automatically after Finish? Can I set the drivers manually?