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Hi Kevin,

Our development team have said that it's to stop a type of attack where someone can change invoices on your account from other websites that you visit. We generate a security token when you view an invoice which must be used to perform any action on that invoice (paying, voiding etc.). There's a limit to how many of these tokens we store, so if you open too many invoices at the same time, then you won't be able to change some of the invoices you opened first without re-opening them.

Apologies for any inconvenience this may cause.

Let me know if this raises any further questions.