paths or require_once doesn't work after enable image and image_attach

Hallo,
I tried image and image_attache (image.module) localy and it worked fine. Then I tried it on a server and something really bad happened:
My scripts which are called by require_once in a page type node didn't work any more. I couldn't find the error.
To solve this, I disabled image and image_attach. But (what a shock) it didn't work now. Finally, I replaced most of the require_once commands by require. Now it works again.
But because I don't understand this error I don't dare to try this module again.
Please, does somebody have an idea if this is a bug or what I do wrong?
Thanks,
H.