The prototype of the function _error() in mysql.php backend seems to have been altered and internal calls in source code have not been updated.

The errors happening in mysql backend are not logged (an empty line appears in error.log)

The new function prototype is function _error( $query, $fname, $error ) and some calls use the prototype $this->_error( $message )

for example :
L448 $this->_error( "The fetched offset value '$expectedOffset' does not match the computed one for the file '$filePath', aborting fetch." );
L458 $this->_error( "The fetched offset value '$expectedOffset' does not match the computed one for the file '$filePath', aborting fetch." );
L497 $this->_error( "File '$filePath' does not exists while trying to fetch its contents." );
...