Hello there, @tjones.

Thanks for reaching out to us here in the Community. I'm here to help fix the error you're getting when modifying a custom report in QuickBooks Online (QBO).

Just to confirm, may I know what specific report you're trying to customize? In the meantime, let's try to perform some troubleshooting steps that can help isolate this. To start off, log into your QBO account using a private browser window. Your regular browser's settings or old cache might have caused this unexpected behavior while using QuickBooks.

Here's how:

• On your keyboard, press Ctrl + Shift + N (Google Chrome).
• Ctrl + Shift + P (Firefox or Internet Explorer).
• Control + Option + P (Safari).

Once logged in, go back to the report that you're working on and try to modify or customize it again. For your reference, you can check out this article for more information: Customize reports. If it works, go back to your main browser and perform clear cache to remove temporary internet files on your computer.

The steps to do this will differ depending on which browser you’re using. For further guidance, please visit this link: How do I clear my browser cache and temporary Internet files?.

That should get you fixed right up. Please let me know how it goes by leaving a comment below. I'd be glad to answer should you have some additional questions. Have a good one and advance happy Thanksgiving!