An Update to my previous post. This evening I spent some time trying to figure it out and haven't come up with any solutions. I ended up installing a default zen cart (no custom templates or extra plugins) v1.5.5f into a new directory (/dev) and the same issue happened.

The redirect blank screen and url I am seeing after logging in from clicking the green authorize button is:

https://mysite.com/dev/square_handler.php?code= (an OAuth that is different than mine that changes with each attempt) &state= (set of random numbers that also change after each attempt)#_=_

Not sure if that is any indication, but that is where I am at currently.

As stated earlier everything was working just fine a few days ago. I wanted to clean up the sites back end and database. So I created a test directory, installed a clean v1.5.5f and new database. Installed all the plugins and template I use, got it back up in running and transferred it to the root directory. The only thing I did not set up was the payment modules, which is just paypal and square, until the updated site was on the root directory. Paypal set up just fine and payments process during checkout.

Nothing was changed on my host or in my square account. I did try creating a new square app, but same issue and url happened.

-Steve