When updating WordPress from 4.2 to 4.2.1, the process stops at Enabling Maintenance Mode…

Thankfully, it doesn’t actually end up in maintenance mode, but the update doesn’t happen.

The problem is caused by a bug that will be fixed in a future release (likely 4.2.2 from the looks of it).

The problem is, now the regular push-button upgrade process doesn’t work. So you will likely have to do a manual upgrade. Or, you can edit the file that has the bug in it.

To do that, modify wp-admin/includes/file.php replacing line 149:

if ( empty( $filename ) || '.' == $filename ) {

with

if ( empty( $filename ) || '.' == $filename || '/' === $filename ) {

After updating to 4.2.1, you may need to make this edit again as it will get overwritten.