@Daniel ... Is that the error ...
"CREATE TEMPORARY TABLES ... blah, blah"
If it is, could be the DB user you have setup for your Moodle (in config.php file of the code) doesn't have the privileges (permissions) to create temporary tables.
See @Eloy's response in this rather old thread (probably need to start a new one).
And other responses (not to your posting but in this thread) that suggest turning on debugging to see what Moodle says (always a good idea).
Let's see if there is anything of a try at a backup ... got anything that can browse files on your Moodle server? Use that tool and look in the data directory of your Moodle (path to it found in config.php of the code).
Example is for Linux but should be similar on Windows:
The data directory for the Moodle is in /var/www/moodledata/
In there, a 'temp' directory and in temp a 'backup' directory.
So /var/www/moodledata/temp/backup/
Succesful backups leave a 0 byte log file and the name looks something like this:
18eb07f85bb26c9a6f95f3684972125a.log
A failed backup, leaves a directory, like:
998ef86bf276620010c16f88d75c6624
Look in failed backup directory and see if there is a moodle_backup.xml file or even a file that has 'backup' in front and the extension is .mbz.
*IF* there is a 'backup-blah-blah.mbz' file, the completion of the backup failed at a stage when the code tries to *copy* the .mbz file to filedir.
IF there is no temp files then suspect this might be related to the first ... ie, priv's of the DB user in config.php that doesn't have permissions to create temporary tables.
Check with whatever tool you have to look at the database server and users. Can't tell ya how to do that specifically.
'spirit of sharing', Ken