I have just resolved the update "loop" problem; I'm running WordPress on a virtualized Linux server, with Apache Web Service, PHP and MySQL.
Thanks to jeffpurcell for the hints... After verifying that my server component services were up to snuff, turning on debug, and increasing the PHP memory space in /wp-admin/admin.php .... still no help... so I consulted with the most excellent tech support folks at Rackspace, (my hosting service), who quickly diagnosed the issue:
On my server, the WordPress update was asking for ~36MB of memory, but:
Regardless of the admin.php 'memory_limit' setting, Linux imposes a
default session memory limit on PHP of 32MB
If you've tried the memory limit increase in wp-admin/upgrade.php to no avail - you need to go "outside" the PHP box to your server's PHP service settings...
I suggest you contact your hosting service to make the following changes, however; if you do have shell/root access to your Linux server, you can increase the PHP session memory limit temporarily by editing the vhost.conf for your domain.
Once you have successfully updated WordPress, make sure to restore the php_admin_value memory_limit to default settings - otherwise your server will quickly run out of memory!
Keep in mind, this example is for a virtualized Linux server, with WordPress running in a subdomain of the virtualized domain-name.com ... so change the paths to suit your own setup:
Edit the file:
Put this in it:
php_admin_value memory_limit 128M
Run the following command via SSH:
Run the upgrade, and then remove what you added to the vhost.conf file. After removing the file, run the above command again:
I have now successfully updated WordPress - and all is (seemingly) well. With DEBUG turned ON I am seeing a slew of notices regarding various plugins - another post topic.
Hope this helps!
Key West Computer Guy