There is a problem backuping 255.43 MB db due to memory limit.
Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 10747858 bytes) in./functions.inc.php on line 671
If the memory limit for PHP or the PHP timeout value is to low, fails to backup large databases.
Therefore increase the memory limit and the timeout value to backup these large databases,
There are several ways to do this - please have a look into the PHP help: