marketpress paypal error

Hi,

having this error when purchasing via paypal checkout:

There was a problem connecting to PayPal to setup your purchase. Please try again.
10002 - Security error - Security header is not valid

(filled in api signature correctly)

what went wrong?

thx 4 ur support!

PS. asked this question also here: https://premium.wpmudev.org/forums/topic/upload-file-for-downloadable-products-not-working#post-266224