Garbled output after saving mailbox configuration

I'm not sure if this has been reported, but I found only some similar bug reports.
I have a fresh Mailhandler installation with
drupal 6.25
Autoload 6.x-2.1
Mailhandler 6.x-2.4+7-dev
ctools 6.x-1.8
Job Scheduler 6.x-1.0-beta3
6.x-1.0-beta11
PHP 5.2.16
Theme: Zen/Rockclimbr

1) When I create a new mailbox (or edit a existing one) I get a get white screen with the following text when I click on the save button:

{ "status": true, "data": "\x3cdiv id=\"mailhandler_test_results\" class=\"form-wrapper\"\x3e\x3cdiv class=\"messages status\"\x3eMailhandler was able to connect to the mailbox.\x3c/div\x3e\x3cdiv class=\"messages status\"\x3eThere are 0 messages in the mailbox folder.\x3c/div\x3e\x3c/div\x3e" }

But when I go back to mailbox overview page the mailbox has been created/updated successfully.

2) When I try to add a new mailbox and make an mistake (for example forgot the administrative title) I get a screen with the complete html code of the page, but not a rendered page. After going back and correcting the error the mailbox is saved and I get the text from #1.

I can reproduce this behavior on a second site with the same modules but the omega theme.