1.3.9 --> 1.5.1, customer login loops back to login page with new password, why??

Testing 1.3.9h to 1.5.1 upgrade, no templates applied.
Appears all data came in fine etc., no apparent issues during database upgrade.
Add product to cart, try go to checkout, customer login does not allow login, instead it LOOPS BACK to the login page.
Requested new password, put this in, still loops back to login page.
NOTHING in the /logs folder.
How do I fix this?? Google doesn't give any good results.
This is on the STORE side of things, not admin.
Really don't want to redo the whole database!!!
Thank you, Tom

P.S. Additional testing: add a product, goes to cart, click go to checkout, login, loops back to CART!! Why??

Zen Cart 1.5.1

Database Patch Level: 1.5.1

v1.5.1 [2013-05-02 19:49:35] (Version Update 1.5.0->1.5.1)
v1.5.0 [2013-05-02 19:49:33] (Version Update 1.3.9->1.5.0)
v1.3.9h [2010-12-21 22:30:33] (Version Update 1.3.8->1.3.9h)
v1.3.8 [2008-03-09 11:32:58] (Fresh Installation)
PHP Version 5.3.22

Server OS: Linux 2.6.18-308.24.1.el5 Database: MySQL 5.1.68-cll
Server Date: 05/02/2013 20:09:12 Database Date: 05/02/2013 20:09:12
Server Up Time: Disabled/Unavailable HTTP Server: Apache
PHP Version: 5.3.22 (Zend: 2.3.0) PHP Memory Limit: 128M PHP Safe Mode: Off
PHP File Uploads: On Max Size: 20M POST Max Size: 20M
Database Data Size: 1,128 kB Database Index Size: 626 kB

Solved -- partially/mostly, I think.
Was testing with Google incognito mode, something was off with cookies or something...
Tested with two other different browsers, things worked.
Thank you, Tom

Ya, those symptoms are all cookie/session related. And "incognito" or "private surfing" mode breaks cookies.

Ref: http://www.zen-cart.com/content.php?...working-for-me