No link to Paypal to pay hosting plans


When a customers tries to pay his hosting plan, the normal Paypal gateway is not reacting (only the Paypal Express checkout - but this don't accept recurring payments). See the video here:
But when I'm using the WHMCS directly without the plugin everything works. So it seems to be an plugin error.
WHMCS support gave me the following error message:

Unfortunately we do not support the integration plugin you are using.

I can however point you to the errors I do see:

Uncaught SyntaxError: Unexpected string pwstrength.js:55
Uncaught ReferenceError: showStrengthBar is not defined ?a=view:936
(anonymous function)
==== END QUOTE ===

These are javascript errors on the page.
So could you help me urgently.