FrankDeBlock-Burij said

in the notification bar "to protect your privacy, Thunderbird has blocked external content in this message" used to be an "Options" button, this was very useful to view external content

Do you have View/Message Body As set to Original HTML?

yes it is set to Original HTML

If you open Tools/Options/Advanced/General/Config. editor, what is the value for the preference permissions.default.image? It should be 1.

permissions.manager.defaultsUrl standaard string resource://app/defaults/permissions

No, the preference to look at is permissions.default.image.

Also, open Tools/Options/Privacy and make sure 'Allow remote content in messages' is not checked, and in Exceptions, there is no entry for the site for which you can't see remote images.

permissions.default.image standaard integer 1

'Allow remote content in messages' is not checked

in Tools/Options/Privacy exceptions I see only accepted sites

You appear to have the correct settings, so I wonder if the usual Options button is missing due to a broken layout/toolbar. To test this, run in safe mode (hold Shift when you launch TB) and see if it makes a difference.

we seem to be nearing the solution: in safe mode, the button "options" appears but now I miss my Lightning Agenda

Restart in safe mode, but this time check 'Reset toolbars and controls' and click 'Make changes and restart'. That should fix it, unless you have an incompatible add-on, other than Lightning, or non-standard theme.

http://kb.mozillazine.org/Safe_mode#Safe_Mode_options

Reset toolbars did not work

then I have deactivated all Add-ons, except for Lighning and Provider for Google Agenda an Bullguard Anti-virus

now everything works as a whistle !!!

many thanks, Frank