Hi again,
The server at userpage.fu-berlin.de indeed seems to impose narrower restrictions than the one at amor.cms.hu-berlin.de
It is true that https://userpage.fu-berlin.de/jonesanam/stimuli/ is not publicly accessible, but the images you upload there are publicly accessible (see, for example, https://userpage.fu-berlin.de/jonesanam/stimuli/1erbsenradiopfanne.png)
The PHP script’s address, however, returns a 500 error: https://userpage.fu-berlin.de/jonesanam/audios/saveRecordings.php. It might be that they refuse to serve PHP scripts altogether, or that something is wrong with part of the configuration somewhere. It’s really hard to tell, a 500 error is very generic. You said you made sure your PHP file is executable (you could try permission code 0644)?
Jeremy