Impossible to add new content
review permissions in the /sites/all/files directory. you can test inside files directory, changing permissions to test the correct permissions. for example you, for test, can change to 755 in recursive mode: chmod 755 files/ -R. also review the owner of directory. According my experience, the owner can be a cause.

(According I remember, permission 777 number is the most higher number, but not is recommended).