I have hit the same error on a SharePoint 2010 Enterprise and a MOSS implementation. The cascade works as expected but when I attempt to save the form, the data does not commit and I get the standard Error page with troubleshooting. I added a new button
(for testing) to the form with a simple _commit command and recieved the same error.
When the code is removed, the forms saves as expected.
For the 2010 implementation, the code was added via a content editor webpart.(I know it is not recommended) but did not have Designer available at the time I added to script. For the MOSS Implmentation, I added the code as recommended in the documentation.
Both Custom forms were created via the standard processes recommended with SharePoint Designer. Versions of SPServices and JQuery are indicated in the script. I have stared at the code for an hour and cannot see anything wrong..
Does anyone have a suggestion or thought? I expect this to be a V8 moment... but wanted to ask.. Thanks in advance.