Hi all;

I'm getting crazy. I've been trying to get PP express running but I consistently receive a 10401 error in both live and sandbox modes.
I've ridden all posts regarding this issue and can't find anything that works. For other posts, I can see that the following information is relevant:

-I only have one active currency, which is mexican pesos (MXN)
-Default language is spanish. I'm keeping english only for admin purposes
-I can see that the [AMT] value sent to PP is always 0 (reading the paypal logs)
-Thousands separator: ,
-Decimal separator: .
-Decimal positions: 2
-Symbol at left: $
-Symbol at right: (none)
-Value: 1
-Instant payment notifications is enabled (@ PP)
-ZC: V. 1.3.8.1

Just for discard any corrupted or misconfigured file, I made a second, fresh installation of Zen Cart and set up the basics, getting identical results.

PLASE HELP before I get nuts!!!