Finally I found why an admin extension was giving a blanc screen on the home page of a webshop.
Nothing wrong with that extension, just some changes in some admin programs in ocmod.
But the strange thing is without that extension no notice errors are logged in error.log. When I install that admin extension, than notice errors (from different programs) are logged in error.log
First showing errors was set on, so with that admin extension the notice errors were also displayed and that was giving a blanc screen because of compression set on (no echo's allowed before sending headers).
So the question is now finally, why are notice errors never logged but only when I just install that admin extension in ocmod. I see that this extension have also changed two language files, I see that changed files in system/modifications but not in vqcache. Is there something going strange in ocmod, or do you see not always all the files in vqcache ?