The solution is to change the type of the column post_parent:

cd "C:\Program Files (x86)\MySQL\MySQL Server 5.1\bin\"

mysql.exe –u root -p
Enter password: ***********************

mysql> use MyWordpressDatabase
Database changed
mysql> ALTER TABLE wp_posts CHANGE post_parent post_parent BIGINT;

See: http://ardentdev.com/fix-for-wordpress-xmlrpc-500-internal-server-error/