After upgrading to joomla 3, I suddenly got this error.
Seems like the class name of the view class cannot contain uppercase letters anymore, except in the word View.

E.g.

My view class was named like this in joomla 1.5:

PaypalRegisterViewPaypalRegister

I had to change it to all to lowercase except the “V” in view, like so:

paypalregisterViewpaypalregister