by Guillermo Madero.
Hello Ulrike,
You may need to increase the limits of the max_execution_time, post_max_size and upload_max_filesize directives at the php.ini configuration file.
Please see:
http://docs.moodle.org/27/en/Installation_FAQ#Maximum_upload_file_size_-_how_to_change_it.3F
http://docs.moodle.org/27/en/Administration_FAQ#How_to_change_the_maximum_execution_time