Hello there, seb2.

There are times that your default browser is filled with data and temporary files from previously visited websites. With this said, I recommend using a private browser. Doing so helps isolate the issue. You can use the keyboard shortcuts below:

• Chrome: Ctrl+Shift+N
• Internet Explorer: Ctrl+Shift+P
• Firefox: Ctrl+Shift+
• Safari: Command+Shift+N

Once you've opened a private browser, try to create a new invoice again. If it works, to ensure this doesn't happen again, it's a good idea to clear the cache of the browser you're using or trying other supported browsers. 

I'll appreciate if you can get back to me and share how it works. I want to ensure that this gets resolved. Have a great day!