WHMCS Integration: 500--Internal Server Error

The plugin shows "WHMCS Integration: 500—Internal Server Error" whenever a client tries to pay (in any way) or view invoice or complete an order! Upon order "completion", WHMCS creates the order as pending (which is normal) but the client see only the error.

No apache errors.

I have installed whmcs in a subfolder named "clients" in public_html, WP also lives in public_html.

Any suggestions????