To ensure that I'm on the same line, may I know the specific error message you're getting when creating an invoice or what happen every-time you initiate invoice? This will help me provide the best resolution for the issue.
In the meantime, have you try login into a private browser (incognito). There are times that the browser is full of frequently accessed page resources, causing some unusual responses.
We can start by accessing your account in a private browser. This mode doesn't save your searches, visited pages, log in details, and cookies.
Here's are the shortcut keys:
Ctrl + Shift + P (Internet Explorer or Firefox)
Command + Shift + N (Safari)
Ctrl+ Shift + N (Google Chrome
Then, let's go to create the Invoice to check if it's working well. If everything looks fine, go back to your regular browser and perform a clear cache. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system, and you'll be able to work with clean trash. However, if the issue persists, try using other supported browsers.