[TYPO3-english] Sysext form shows error on Form Wizard

[Ok Z4uit]

Hello all,

Yesterday I wanted to create mailform via de Form Wizard (a test, so I was sure editors did not have to use typoscript) and I got the following error:

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.

As far as I know, I never intentionally did any of those three things.
My question, how can I re-install the system extension form or fix this error?

Best regards,

Adriaan