PayPal express button v1.3.8 not working

I've been having problems with the PayPal Express module. I installed it quite a while ago and have been using it successfully, but it has been failing recently.

Sometimes, when a customer checks out, I receive the order in zencart, but no payment via paypal. Instead, the payment is reported as "check/money order", even when I have that payment module disabled. I haven't been able to recreate that failure mode myself.

I have been able to reproduce a different failure mode. If I check out by clicking on the "checkout now" button, log-in, or create an account under zencart, and select paypal as the payment method, then everything works. If I click on the "checkout with paypal" button, then it doesn't work. I get redirected to PayPal, login, go through the payment process there, and then the process comes back to zencart, through steps 2 and 3 again, and get directed to PayPal once more. At that point, however, there is no further navigation. There are no payment prompts and no redirection back to my webstore. The order doesn't get placed and no payment is made.

I'm using version 1.3.8a and really don't want to have to upgrade right now. I've made core code changes and haven't looked at whether or not they'll work in later versions.

Any ideas?

Thanks,
Eric

There were problems in v1.3.8 that could trigger those symptoms, which were fixed in 1.3.9