Anyone know how to empty the cache on a MacBook Pro? A rep from my bank has advised it as..

[Ocelot II]

4. I have one, and never heard of problems accessing an online account

because the cache (temporary files) is too full. But you could try this:

How to clear the application cache on Mac
Open Finder, select the Go menu, and click Go to Folder.
Type /Users/[YourUserName]/Library/Caches in the window and click Go.
You'll see the user caches for all your apps. You can go into each folder, select the files inside, and drag them to the Trash.