Forbidden when returning from paypal

[holocubierta]

Hi there,

We are developing a new store and all seems to be correct, but when the payment is completed (I receive all emails correctly and the amount is charged/received in paypal accounts) user is redirected to this page:

mydomain.com/index.php?option=com_virtuemart&view=vmplg&task=pluginresponsereceived&on=eeb308&pm=1&Itemid=243&lang=

And it shows a 403 Error Forbidden

The point is that if I copy and paste that adress in the browserbar it works! But, obviously I need it to work when it comes directly from Paypal.

What am I doing wrong?

Virtuemart 3.0

Thanks!

EDITED: the eeb308 in the link is the order number.

EDITED2: As it is a test, the whole web is in a subdomain (sub.mydomain.com)

[GJC Web Design]

the return url appears correct so have you asked the host what the 304 error is logged as? 
perhaps security firewall problem on the server?

[holocubierta]

I will ask them, but when using Paypal Express it redirects correctly and no-error is shown.

[GJC Web Design]

that's why I say it could be a server mod_security thing or firewall