Hello there. Another Moodler had this problem and found it was an issue with space - here is the forum thread https://moodle.org/mod/forum/discuss.php?d=221857 Perhaps you might like to try this?
thankyou for the answer!
I have not space issue because the server is mine and I can access directly to the hard disk and to the directories.
And there is not also problems of permissions because I give total control to "everyone" user to the moodledata directory.
Is there another possibility?