From b2evolution manual

If you are reading this, you probably encountered a file permission error within your /media directory.

The /media directory is the directory on your server where b2evolution will store all the files/images you upload to your blog.

Apparently, b2evolution/PHP made an attempt to read or write a file within the /media directory but PHP failed due to UNIX/Linux file permission restrictions.

[edit] How do I fix this?

Some hosting providers are set up in a way where PHP will never have this kind of problem. Others are set up in a way where PHP will have this kind of problem every time it tries to create or modify a file or folder. The best solution to this problem depends on your particular host setup.

• To fix this yourself, your will find helpful information in: Directory and file permissions. There is a useful script in the debugging page that helps you determining the current situation of the permissions in the /media/ folder.

• You may also request help from your hosting provider's tech support to achieve this. That is what they are here for! ;) Tell them "My PHP application (b2evolution) needs write access to the /media directory and its subdirectories but there is a permission error. How do we fix those permissions?". If a complete file path is displayed (example: /var/www/user/web/media/) give them that path also.

[edit] More info about error messages

[edit] "The blog/user's media directory «xyz» could not be created"

b2evolution (actually PHP) has no permission to create the directory needed for storing the media files belonging to a certain blog or user. Make sure all subdirectories of the /media folder are writable by PHP.