Paypal-Sandbox-Mode does not work

Hi, i am using the Standard-Payment-Plugin "os_paypal". On the right side of the screen i can input my parameter. The live-mode works very well, no problems. But i cant solve the problem using the "test-mode". I am doing the following steps:

1. Chance paypal mode to "Test Mode"
2. Enter the Paypal Sandbox Email, in my case: sb-gi4jq18290445@business.example.com
3. Going to "Advanced-Tab, filling in the "NVP/SOAP Sandbox API credentials" paypal is providing me.
 Sandbox API User: sb-gi4jq18290445@business.example.com
 Sandbox API Password: 5SL7LKWV9SV66TKQ
 Sanbox API Signature: khil9oXb6TWDZlcDkVufCSg-Jo4AxrhzksZU
 (Note: that are not the real parameter from my account)
4. In paypal i am activating the sanbox-mode, of course

When i now want to order a ticket, the system wants to redirect to paypal, but then the paypal-error-message "Sorry, there is an error, please try again later" is showing (i translated it from german.

Do you have an idea, what i am doing wrong? I didnt found another thread for this topic. I attached two screenshots