PayPal Express Checkout log in loop with no final sale

In debugging this issue, I found that if a Product - General has option attributes it goes into a loop of logging in at the site, then loggin in at PayPal, then clicking PayPal's "pay now" button and being brought not to the site's "thank you" page, but instead to the site's "create account" page and if you create a new account (mind you, you already have one, and were already logged in to it), you are taken back to PayPal where you log in and pay and are then taken back to the site to the create account page again, and so on ad nauseum. And no order is logged in the database.

I also tested on Document - Product with attributes, and it worked fine. Also tested Product - General and Document Product without attributes, and it worked fine.

You have failed to provide basic information about your install

When you reply - scroll up and read posting tips and provide the information

Sorry - I thought that selecting the version (1.5.0) in the drop down menu when I posted provided that info. It is a new 1.5.0 install, copied from a staged site, using cPanel.

And how about the rest of the information
what addons
what php
what mysql
etc...

Add ons:
Image Handler 4
Zen Lightbox
CKEditor
Group Pricing
Multi-Cross Sell
Monthly Sales and Tax Summary Report
ZX Slideshow

PHP Version: 5.3.10

MySQL 5.0.95

copied from a staged site
Did it function correctly before??

This is a brand new, just launched site. If you mean, did it work on the site I copied it from (that I have staged on another domain) - I am unable to test it there. I do not personally have a PayPal account configured for Express Checkout. So I don't know. And my other clients who have a version of this same site and use PP Express Checkout are not using option attributes - so for all I know it doesn't work for them either.

If you still have the "staged" site, FTP files to your PC and re-upload them to the actual site
Leave cpanel out

kobra:

If you still have the "staged" site, FTP files to your PC and re-upload them to the actual site
Leave cpanel out

Unfortunately, I do not have the exact one. But I have one that is similar. What, specifically, should I upload? Or, what can I safely upload that won't wipe out things that are different between the two sites? Config file, obviously. Template files. Template overrides. Anything else?

If you have followed the template override system and do not have any addons that affected "core" files
Then you can load all files from a fresh distribution

I FTP'd all the files and it still is not working properly.
In testing some more, I found that the loop described in my first post happens if the customer doesn't click the PayPal Express Checkout button on the shopping cart page, but instead clicks the normal checkout button and then either logs in as a returning customer or creates an account, gets to the payment option page and chooses PayPal at that point.
So I have temporarily fixed this by changing the button_checkout.gif image to say "Pay by Check", which should result in all PayPal customers clicking the Express button and going immediately to PayPal. Those transactions complete properly.
This is just a temporary "fix", however, and I am still hoping someone here can resolve this.

I can not duplicate this in a stock install...

Care to guess which of the add-on modules I installed could be the culprit?

Again, the add-ons are:

Image Handler 4
Zen Lightbox
CKEditor
Group Pricing
Multi-Cross Sell
Monthly Sales and Tax Summary Report
ZX Slideshow