Troubleshooting PayPal

Selecting PayPal reloads the page

This is a tough issue to diagnose because there's no error. However our support is fairly confident the issue is that your server doesn't support Support SHA-256.
If we look at the API response directly (this would require modifying core SI files so take our word for it) the error is given: error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure
Sprout Apps does not have any experience in resolving this issue, however this support article's answer may help you -  http://stackoverflow.com/questions/34926940/php-paypal-error-14077410ssl-routinesssl23-get-server-hellosslv3-alert-hands/34933173#34933173

Security header is not valid

This is a PayPal API error which signifies your API credentials are invalid, check to make sure that you have the correct credentials for the correct mode. PayPal doesn't allow you to use your live credentials within sandbox mode. If this doesn't help solve the error you will need to contact PayPal, we've seen 1/100 times an account needs to be manually approved by PayPal for API access first.

Invalid Configuration: This transaction cannot be processed due to an invalid merchant configuration. (Error Code: 10501)

This error is the result of an issue with your PayPal account. Please contact PayPal support about this to get it resolved.

Before you do that though, you should confirm your configuration, making sure you're using the appropriate credentials for the proper method (e.g. live/staging).