Lightroom, Windows 10 and "Unexpected error opening catalog"

If you are using Adobe Lightroom 5 and have recently upgraded to Windows 10 (as 25 million people did), you may have seen this dreaded message while opening Lightroom, saying it can't open its catalog due to "unexpected error".

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Pano-Summer 2014.

Panoramic Summer 2014.

Autumn 2014.

September - October 2014.

December 2014. (3 new items)

Early mornings in December

Introduction: Pentax SFXn

First roll of film in Pentax SFXn

Introduction: Pentax MX (2 new items)

First roll with freshly revived Pentax MX

Random photos with Spotmatic F

Random photos with SP-F. May 2015.

Introduction: Canon AE-1

First roll with Canon AE-1

Introduction: Pentax MX (3 new items)

First roll with freshly revived Pentax MX

Introduction: Pentax MX

First roll with freshly revived Pentax MX

Infra-red Summer 2015.

Infra-red summer 2015.

At the quarry, March 2014.

At the quarry, 2014.