warning: fopen(): open_basedir restriction in effect

Hello,
I geht the following warning when updating ID3 Tags of an audio file:

* warning: fopen(): open_basedir restriction in effect.
File(/tmp/getID39yHyUN) is not within the allowed path(s):
(/var/www/xxx/html:/var/www/xxx/phptmp:/var/www/xxx/files:/var/www/xxx/atd:/usr/share/php:/var/www/empty/xxx)
in /var/www/xxx/html/sites/all/modules/getid3/getid3/write.id3v2.php
on line 92.
* warning: fopen(/tmp/getID39yHyUN): failed to open stream:
Operation not permitted in
/var/www/xxx/html/sites/all/modules/getid3/getid3/write.id3v2.php on line 92.
* The following errors were encountered, preventing the ID3 tags from being saved:
o WriteID3v2() failed with message(s):
+ Could not open /tmp/getID39yHyUN mode "wb" -

In filesystem settings I set tmp-dir to "sites/default/files/tmp". This one and "sites/default/files/audio" exist and have rw permissions (765).
Download is public.

I already had a talk to my webhoster and he tolds me it should work because "/var/www/xxx/html" is allowed. At the moment I think it has something to with an access error when browsing directly to "www.example.com/sites/default/files/audio/anyfile.mp3":

Forbidden
You don't have permission to access /sites/default/files/audio/anyfile.mp3 on this server.

I can't explain and I'm researching since hours. Does anybody have an idea?

Thanks,
Haggins