I’ve updated from 1.4.9 to 1.5.4, installing each version between one by one so I didn’t miss the peculiarities of 1.5.0/1.
Now, when I try to view an invoice, old or new, I receive an error. The error is not very helpful as it says a connection timeout but it appears within 1 second so I believe this error message is a default from the shared host that our site is on.