If you're getting this error when creating an expense in QuickBooks Online (QBO), the browser may be already full of frequently accessed page resources, causing some unusual responses.
To fix this, try signing into your QBO using a private browser (incognito). Here's how:
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once logged in, try to create an expense transaction to double-check. This can be done by clicking the + New option and selecting Expense.
If the error is gone, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.