Just to follow up on this in case anyone else stumbles on this thread - I had two vouchers in my account and was trying to book 2 x return tickets. I was getting an error when I selected the option for 2 people/outbound or inbound, but when I switched to booking as a simple return for one person the booking went through just fine, and then I used the other voucher to book the same for the second pax.

I have an AerClub account (i.e. on the avios.com platform) which has kept the avios.com booking engine open as an option for me; I could see and use my Lloyds vouchers in there.