[TYPO3-english] 404 errors in UI buttons TYPO3

[Ok Z4uit]

Hello all,

Last summer I posted a question about 404 messages that appeard using the Form Wizard.
(http://forum.typo3.org/index.php/t/205342/sysext-form-shows-error-on-form-wizard)
I did not get any reaction and it was not so urgent, so I left it as it was.

Now however, the same error appears for the Powermail extension when I click the big black + under "Choose a Powermail Form".
It seems that for all UI buttons I get the message.
This is the error message:

Server error

404 - File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

Is there any possibility to fix this? 
Can I do a repair or do I really have to reinstall and configure everything to get rid of this?

The website runs on TYPO3 6.2.9 and PHP 5.4.37.

Best,

Adriaan