2nd Monitor not working

I have an Acer Aspire All-in-ne running Windows10, with a Dell E228WFP monitor attached.
Device Manager  recognises the new monitor and says everything is working fine.  I also updated all the device drivers, which on reading some of the forums, may have been a mistake.  The Windows system settings display also recognises 2 monitors.
The Dell monitor works fine on a laptop running Windows 10, so I assume the hardware is fine.
The problem is the Dell monitor goes into Auto Detect mode, displaying the Dell logo, and then to  Power Save mode.  The power light changes from green to orange in the process.  Also, the menu and settings buttons on the monitor do not work when connected to the All-in-one but work fine when connected to the original Dell desktop.
All very frustrating.  Any help would be much appreciated.  Many thanks in advance.

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