Error in service activity form

Hello,

since some time I get this odd onload event error when selecting a service in the service activity form.

My error is in german but I'll try my best to translate it:

Error in the customized event of this field.

Field:serviceid

Event:onchange

Error: Can't obtain the attribute "SetParamter" of an undefined or null reference.

Auf deutsch:

Fehler im benutzerdefinierten Ereignis dieses Feldes.

Feld:serviceid

Ereignis:onchange

Fehler:Die Eigenschaft "SetParameter" eines undefinierten oder Nullverweises kann nicht abgerufen werden.

The version of CRM is CRM 2011 Rollup 11

We can't update any further at the moment because we're having some issues which have to be resolved first.

I hope someone can point me in the right direction.

Thank you in advanced.