Yeah since back in some month we took over InvoicePlane from the old Maintainers there also have been some data loss. And you are right, the Demo is atm not publicly accessable.
To make it again available for everyone we have to implement a automatised script which will reset everything once all 30 min or something like this. Untill we do not have this we will not share the login datas and the demo in first place will emain for internal tests.
Sorry to say so.
P.S.: try it out its an awesome piece of FOSS Software which will most probably statisfy your needs.