@Tony Zuko go to Customization > Scripting > Scripted Records and you can disable all scripts there for the record, and renable one by one to identify if one of them is causing this

Thanks! I disabled all scripts and workflows, and removed all formulas from Transaction Body Fields, but the error is still happening. I submitted a support ticket.

that is good safe guard to turn off all the scripts and workflows but normally when you get this kind of error it tells you script ID either in the error or on the URL, so start with that rather than breaking something which is not broken.

We didn’t get any script ID sadly! The answer was indeed to disable all scripts and workflows—I just missed one workflow when doing so the first time. The workflow had a saved search criteria using a saved search that was using an invalid field value, so the error was getting passed from the saved search results, through the workflow, all the way to the transaction confirmation.