Misfunctioning PayPal button in the Contribution Page

There is a problem with inserting a PayPal donation button in the Contribution Page. The button is a html code generated by PayPal and embedded in the 'Thank-you Message's html code in the 'Reciept' tab within the Contribution Page. The Preview function shows a correctly functioning PayPal button, but as soon as the on-line contribution is being tested in the Test-drive or Live mode, the button fails to function. When pressing the button (which shows up after confirming the contribution together with the Thank you message), the initial contribution page is reloaded, instead of bringing the user to the specific PayPal donation link...