PayPal account changing

Hello,

I am a new user of WP and I have a working webpage what I am working with. I have a MarketPress PlugIn and I am using PayPal Express Checkout payment.

Everything was working fine, but now I have to change the PayPal details, because I have a new PayPal account. So I went to PayPal and created a new API signature (username, password, signature). I thought it will be easy to change, so a I went on my WP/Store Settings/Payment Settings and changed the details. Unfortunately it doesn’t work :/ When I try to buy something I get an Error Message “here was a problem finalizing your purchase with PayPal. Please go back and try again.
10486 – This transaction couldn’t be completed. – This transaction couldn’t be completed. Please redirect your customer to PayPal.”

Could you help me, what would be the problem?

Do I have to change some other options as well?

Many Thanks