There are times that the browser is full of frequently accessed page resources, causing some unusual responses. As an initial troubleshooting step, try logging into your QuickBooks Online (QBO) account using a private browser (incognito).
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox or Internet Explorer).
Control + Option + P (Safari).
Once signed in, try to print a test invoice again to double-check.
If it works, return to your default 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 a clean slate. However, if the issue persists, try using other supported browsers.