Permissions problem perhaps?
Is your data file area definitely write-able by the web server application. The easiest is to ensure that all the directories and files are owned by the same user/group that the httpd server runs under.
You don't say what you are running, but on Unix something like...
chown -R apache:apache moodle_data (redhat et al)
OR
chown -R nobody:nogroup moodle_data (suse...)
General help
Trouble Uploading
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