Your experience of being unable to access a customer that already exists is sometimes caused by a browser issue. It is normal for web-based platforms like QuickBooks Online (QBO) to gather cache files for faster browsing. However, too much of this can cause such error. I've got a couple of steps to help get this working.
Let's start by using a private browser. Since this doesn't store data in the cache, it the best place to isolate browser issues. Try these keyboard shortcuts depending on the browser you're using:
Google Chrome: Ctrl + Shift + N
Microsoft Edge: Ctrl + Shift + P
Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
Then, test it by creating an invoice and adding the customer. If you're able to do so, I recommend you go back to your regular browser and clear its cache. This will remove the log files and start with a clean slate. Otherwise, you can try other supported browsers to help narrow down the cause of this behavior.