Hello,
after installation I have this error on every page of multisites component:
Warning: is_file() [function.is-file]: open_basedir restriction in effect. File(/data/web/virtuals/7544/virtual/www/administrator/components/com_multisites/admin.multisites.php/extension.xml) is not within the allowed path(s): (/data/web/virtuals/7544/virtual) in /data/web/virtuals/7544/virtual/www/administrator/components/com_multisites/admin.multisites.php on line 42

there is no such as path. File extension.xml doesnot exist and admin.multisites.php is file, not directory.

it is fresh joomla 1.6.3 installation.

my webhosting provider told me, that it is not possible to disable open_basedir restriction, but basically the problem is the missing file, not permission...

any idea how to fix that?

open_basedir specify the directories in which you website can have access.
So if you try access a file outside of the open_basedir values then this is normal that you get this error.

To know the current values of the open_basedir, you can go in Joomla / Help / PHP Information. There you will find all the open_basedir value.

Here the line 42 reported search for a "deprecated" manifest file that should not exists.

I suppose that you are using a JMS version 1.2.54.
If this is the case, update your Jms multisite with 1.2.55.
See the FAQ on how to get the latest version.
www.jms2win.com/en/faq/faq-joomla-multi-sites#cat-122