TYPO3 Core

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?&RTEtsConfigParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=link&srcpath=/dummy-3.7.0/typo3/ext/rtehtmlarea/rtehtmlarea_browse_links.php
http://www.***.nl/dummy-3.7.0/typo3/ext/rtehtmlarea/t3_popup.php?&RTEtsConfigParams=tt_content%3A1%3Abodytext%3A1%3Atextpic%3A1%3A&popupname=image&srcpath=/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[table]=tt_content&P[uid]=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%2Ftypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1&edit[tt_content][1,]=edit

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. Including cleaning the temp files etc. on my pc.
The error happens at an commercial server. At my own test server everything works just fine.
(issue imported from #M886)