[Typo3] unable to open 'insert weblink' and 'insert image'

[typo3 at gosselt.net]

When I click on the 'insert weblink' or 'insert image' button (using
FireFox) a pop-up window opens displaying an internal server error window.

The url of the pop-up windows are in the same order:
 
<http://www.***.nl/dummy-3.7.0/typo3/ext/rtehtmlarea/t3_popup.php?&RTEtsConf
igParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=link&srcpa
th=/dummy-3.7.0/typo3/ext/rtehtmlarea/rtehtmlarea_browse_links.php>
http://www.***.nl/dummy-3.7.0/typo3/ext/rtehtmlarea/t3_popup.php?&RTEtsConfi
gParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=link&srcpat
h=/dummy-3.7.0/typo3/ext/rtehtmlarea/rtehtmlarea_browse_links.php
 
<http://www.***.nl/dummy-3.7.0/typo3/ext/rtehtmlarea/t3_popup.php?&RTEtsConf
igParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=image&srcp
ath=/dummy-3.7.0/typo3/ext/rtehtmlarea/rtehtmlarea_select_image.php>
http://www.***.nl/dummy-3.7.0/typo3/ext/rtehtmlarea/t3_popup.php?&RTEtsConfi
gParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=image&srcpa
th=/dummy-3.7.0/typo3/ext/rtehtmlarea/rtehtmlarea_select_image.php

My server error logs show the following error (both cases) when i try to
open in full screen editor:
Premature end of script headers: php-script, referer:
<http://www.***.nl/dummy-3.7.0/typo3/wizard_rte.php?&P>
http://www.***.nl/dummy-3.7.0/typo3/wizard_rte.php?&P[table]=tt_content&P[ui
d]=1&P[pid]=1&P[field]=bodytext&P[returnUrl]=%2Fdummy-3.7.0%2Ftypo3%2Fsysext
%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1

And the following error when in page editing:
Premature end of script headers: php-script, referer:
<http://www.***.nl/dummy-3.7.0/typo3/alt_doc.php?returnUrl=%2Fdummy-3.7.0%2F
typo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1&edit>
http://www.***.nl/dummy-3.7.0/typo3/alt_doc.php?returnUrl=%2Fdummy-3.7.0%2Ft
ypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1&edit[tt_content][1,]=e
dit

Opening the page in MSIE doesn't work eighter. The rte doesn't load giving
an JS error with the same url as the server error logs.

 

I have tried different htmlarea versions both global and local installed. I
even made a clean install again (with only the rte installed). Including
cleaning the temp files etc. on my pc.
The error happens at a commercial server. At my own test server everything
works just fine.

 

It probably is a server error. But I have no clue where to look for a
solution.