I get the same error on CS3, CS4 and DW8
It pops up during the checkout wizard after I enter the PayPal ID.
The start screen will hang and not proceed forward if the Shipping Services are selected as yes. If the "no" shipping services is selected then it will go to the tax screen, then the PayPal screen (if PayPal is selected as the checkout method) and the produces the onMouse up error after the PayPal email address is entered.
I have the cart 4.1.5 and the Universal Email is the latest version 3.05. I have dumped the WinCache-.dat files, uninstalled - reinstalled the extensions and the same error is produced.