I recently installted quota_manager. Now I get the following error while saving a user:

Fatal error: Call to undefined method Zenphoto_Administrator::getQuote() in /www/htdocs/w00d7133/zenphoto/zp-core/zp-extensions/quota_manager.php on line 93

Does anybody know how to fix this?

You should probably update to version 1.4.0.2
This was just a typo (getQuote() should be get Quota())
and is corrected in version 1.4.0.2