Hi,
It seems that there was some function which was being referred on the Form onsave method.
Points to check:
1. Whether the opportunity form that is migrated is same as the "Information" form or something else. (Verify in Customizations)
2. If there is some other form that is present, does it contain any form save event call. (This will help to narrow down the issue.)
3. Otherwise, Can you add a sample JS file and a sample function to the Opportunity Information form and check if you get the same error again after adding the sample call on save event of the Information form.
4. If you do not get the error, then disable the call and check again. This should remove any extra call that was being referred.
Let me know your progress on this.
I hope this helps to get your issue resolved.
Thanks & Regards,
Anubhav Bajpai