PROBLEM

Update WordPress
Down­load­ing update from htt.…
Unpack­ing the update…
Ver­i­fy­ing the unpacked files…
Prepar­ing to install the lat­est version…
Enabling Main­te­nance mode… STUCK

SOLUTIONS
replac­ing line 149 in wp-admin/includes/file.php
if ( emp­ty( $file­name ) || ‘.’ == $file­name ) {
with
if ( emp­ty( $file­name ) || ‘.’ == $file­name || ‘/’ === $file­name ) {

SORUCE : https://wordpress.org