hi, I've had the same problem, what you can do is either back up the program then once on a usb open up the folders and find the invoice folder and delete old invoices once you've do this open express invoice and restore for the amended usb.
or you can open express invoice first thing in the morning and click on invoices the page will open and then take a while to let you do something like look at this week or yesterday but once this has run its coarse the system should work normally all day.
hope this helps it's not ideal but it works