Drafts can no longer be converted to invoices

For projects I always create a draft invoice and enter work don on a daily basis. When I'm ready I convert it to an invoice and mail it to the client. But for the last two invoices this system didn't work anymore. When I try to convert the draft into an invoice the system says something about "duplicate name STD00001 is not possible". The only way around is to duplicate the draft and save it. Then I delete the draft that couldn't be converted.
This used to work properly and I have been using it for more than a year so it seems recent changes have broken the system.