Online payment and submition of data

Hello,

After customer completes the rental application on my website and clicks Submit, page changes to Paypal, but says Internal Server Error.

How can I fix that to stay on paypal and make their payment?

Please note: below the Submit button, there is a link that says click here and that takes them to Paypal properly. Issue is when clicking on the Submit button it should say Thank you and direct the customer what to do next. How can I set it up to do exaclty that?

If you follow this guide  you should get the desired scenario. However I've checked the form Rental Application  and I see no Paypal integration in there. Have you removed it?