PayPal Express Checkout log in loop with no final sale

[seattleannie]

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.

[kobra]

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

[seattleannie]

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

[seattleannie]

Again, the add-ons are:

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