After several updates i detected a second installation of Mozilla's email-client Thunderbird.
Something went wrong and so i deleted both to install a complete new version i downloaded earlier through:
mozillamessaging.com
Make sure you delete old version the proper way! Go to configuration -> sofware and select the programm to be deleted.
After done so, i noticed a strange behavior while installing a new version. It kept returning the message 'not removed completely, restart your computer'. I did and i did it again and again. No result, the message kept comming. So where to look now, something must have been left in the register or in the programm folder.
After a short call to John (my supplier) he suggested to have a look in that folder (%PROGRAMFILES%\Mozilla Thunderbird) again. Earlier on i saw one file left
thunderbird.exe.moz-delete. Not executable and appearantly abundant. To be sure nothing could go competely wrong i made a copy of that file, stored it in another folder and deleted the one in 'Mozilla Thunderbird'.
A new attempt to install went smooth and without any problem
Short note: just delete the programm, NOT the entire folder if you want to be able to get to your emails after installing a new version
Update: it should be thunderbird.exe.moz-delete and not 'firefox'