White screen of death in Customise

No problems in my live site topshamsociety.uk

My development site is running on SME Server 9.0 at home. In an ibay, so the wordpress installation is located in servername/topsoc/html/ but accessed from a browser by http:/servername/topsoc/

The white screen of death occurs with Customize, Header and Background.

The live site uses
http://topshamsociety.uk/wp-admin/customize.php?return=%2Fwp-admin%2Fthemes.php
http://topshamsociety.uk/wp-admin/customize.php?return=%2Fwp-admin%2Fnav-menus.php&autofocus%5Bcontrol%5D=header_image
http://topshamsociety.uk/wp-admin/customize.php?return=%2Fwp-admin%2Fnav-menus.php&autofocus%5Bcontrol%5D=background_image

The development site uses
http://servername/topsoc/wp-admin/customize.php?return=%2Ftopsoc%2Fwp-admin%2Fthemes.php
http://servername/topsoc/wp-admin/customize.php?return=%2Ftopsoc%2Fwp-admin%2Fnav-menus.php&autofocus%5Bcontrol%5D=header_image
http://servername/topsoc/wp-admin/customize.php?return=%2Ftopsoc%2Fwp-admin%2Fnav-menus.php&autofocus%5Bcontrol%5D=background_image

I’ve tried rewriting the path string %2Ftopsoc%2Fwp-admin in the browser’s address box to see if adding “html” will bring up the screen, but nothing works.

I don’t know enough about scripting to work out how customize.php uses return=xxxx, so I have no idea what the problem is, other than it has something to do with SME Server and the way customize.php uses the parameter passed by return=xxxxx

A solution would be very much welcomed. Thank you.