Event Reg. - Paypal Standard - 500 Internal Server Error -

This is on a Jooma 1.5.9 - CVCRM 2.2.2 - After clicking on continue in the Event Registration I get the error 500 below and the URL associated with it. Contacted Paypal but no response.
Any help would be highly appreciated. Thank you.

The event details are here:
https://www.community-mediation.org/index.php?option=com_civicrm&view=Events&Itemid=91&lang=en

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@paypal.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.

https://www.paypal.com/xclick?business=brenda%40cmediation.org&notify_url=https%3A//www.community-mediation.org/administrator/components/com_civicrm/civicrm/extern/ipn.php%3Freset%3D1%26contactID%3D1%26contributionID%3D28%26module%3Devent%26eventID%3D9%26participantID%3D20&item_name=Online+Event+Registration%3A+Facilitation+Training&quantity=1&undefined_quantity=0&cancel_return=https%3A//www.community-mediation.org/index.php%3Foption%3Dcom_civicrm%26task%3Dcivicrm/event/register%26_qf_Register_display%3D1%26cancel%3D1%26qfKey%3D289ee3c9f84ebcc2e86d4a8276c598f1&no_note=1&no_shipping=1&return=https%3A//www.community-mediation.org/index.php%3Foption%3Dcom_civicrm%26task%3Dcivicrm/event/register%26_qf_ThankYou_display%3D1%26qfKey%3D289ee3c9f84ebcc2e86d4a8276c598f1&rm=2&currency_code=USD&invoice=3bad3d1be6b0758e4c88b943f37b691b&email=et%40comworks.net&first_name=Eric&last_name=Treu&address1=16+High+Plains+Road&city=Branford&zip=06405&state=CT&cmd=_xclick&amount=25