For some time I have disputed clients who have reported they were unable to view their online invoice. The ïnvoice viewed" ONLY means the client has opened their email. It does not mean they have been able to open their invoice. I do not use the automated invoice numbering system provided in Essentials. My choice has been to use the format of the sequence of their advertising number and the year. ie 12007/2017 .What I didnt know was that by using the "/ "symbol, the invoice could be recorded and emailed BUT NOT ABLE TO BE OPENED by the client. Hope this knowledge helps others. As soon as the / symbol was removed, it worked perfectly. but it took months to find this out.