Hi Ryan
Thank your for your quick answer. I have checked my ubrcart and PayPal account settings.

This is my ubercart setup:
PayPal Website Payments Standard settings
Currency code: CHF
PayPal Website Payments Pro settings
Currency code: CHF
Are there more currency relevant settings in ubercart?

The primary currency of my PayPal business account is Swiss Franc.

Is there a way to debug the communication with PayPal to get more detailed information about what's going wrong?
Regards Martin