Hello there, @cfitzpatrick, 

I appreciate you for attaching a screenshot above.

We've received reports about the error you're getting when removing the unbilled expenses. Rest assured that our product engineers are actively working to have a permanent fix. 

I'd suggest contacting our Customer Support Team. They can add your company to the list of affected users, so you'll get updates via email. They are available from 6 AM - 6 PM PT (Monday - Friday) and 6 AM - 3 PM PT (Saturday). Here's how: 

1. Go to the Help menu at the upper right. 
2. Choose Contact Us. 
3. Enter I keep on getting this error message: "Something's not quite right. Please wait while we are converting Billable Expenses to non-billables.psng534" in the What can we help you with? box. 
4. Click Let's talk. 
5. Select Get a callback or Start a chat. 
6. Enter your contact information. 
7. Choose Confirm my call or Send message. 

Once resolved, I recommend running the Unbilled Charges report to view all the customer charges you haven't invoiced yet. Just go to the Reports menu and locate it under the Who owes you section. The screenshot below serves as your visual guide. 

I'm just a comment away if you have other concerns. Enjoy the rest of your day. Keep safe.