If I want to enter expense input I get a error message?

Welcome to the Community, @pieterschultz188. Let's work together to get rid of the error. 

Sometimes your browser holds too much app data and add-on's that causes unusual behavior of the program. This is why you're getting an error while creating an expense. Let's access your account via a private browser to confirm. 

Here's how:

• CTRL+SHIFT+N for Chrome.
• CTRL+SHIFT+P for IE and Firefox.

Once done, log in to your QuickBooks account and create an expense. If it's working fine, go back to your regular browser and clear its cache. 

If you're getting the same result, try using another supported, up-to-date browser. 

Just in case the issue isn't resolved, I'd appreciate it if you can share the specific error message and number so I can refine my research and help resolve the issue effectively. 

I've also added our page about how to enter, edit, or delete expenses in QuickBooks Online for future reference.

You can always come back if you have questions. I'd be around to assist you.