[TYPO3-english] Bizarre bug: adding a plugin in BE forwards to FE view

[Jennifer Koenig]

I have a really weird bug and I'm not sure what I've done to cause it: upgrading from 6.2.5 to 6.2.9? Setting up a 404 page in the install configuration using a redirect?
 
When I try to add a plugin as content-element to a page in the backend, the form forwards to the front page of the front end, showing this page in the frame. The plugin is not saved. This happens even if I just click "add new content element", "general plugin", give in a name, and save. (i.e., even if I haven't actually selected a plugin).

When I look at the URL that's generated from saving, I see that the front-end url has been appended. It looks like this:

http://mysite.com/http://mysite.com/typo3/alt_doc.php?&edit[tt_content][1760]=new&defVals[tt_content][colPos]=0&defVals[tt_content][sys_language_uid]=0&defVals[tt_content][CType]=list&returnUrl=%2Fhttp%3A%2F%2Flal-relaunch.de%2Ftypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1760%26

When I look at the source code html for the form, pre-saving, it looks look this:

<form action="/http://mysite.com/typo3/alt_doc.php?&edit[tt_content][1760]=new&defVals[tt_content][colPos]=0&defVals[tt_content][sys_language_uid]=0&defVals[tt_content][CType]=list&returnUrl=%2Fhttp%3A%2F%2Fmysite.com%2Ftypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D1760" method="post" enctype="multipart/form-data" name="editform" onsubmit="document.editform._scrollPosition.value=(document.documentElement.scrollTop || document.body.scrollTop); return TBE_EDITOR.checkSubmit(1);">

Is it normal for the form action to have a first, forward-slash before http? 

Has anybody ever experienced this before? Does anyone have any tips about how I might go about debugging this, where I could at least start?