Altering Invoices

Recently we have been unable to alter invoices for over a certain amount (eg. $2000). We invoice for specific items, and some times these are quite extensive (over 50 lines). Should the client, then wish to add to the invoice later, we are unable to alter the invoice - account right crashes and then defaults to an email to inform them of this issue. We have sent numerous emails regarding this issue but are yet to receive a response.

This is a fairly recent development (only started happening a couple of months ago), so would appreciate any ideas on how to fix.

Cheers