I've put it in this forum as I think it's more of a bug report on V5 than anything else.
I've had a new applicant today, I asked a question for clarification and he replied - so I went to upvote prior to responding and got the following error:
Tried again, and got a similar message with a different ID:
I suspect it was because he wasn't, at that point in time, a member of the guild. Once I'd accepted his application, I was then able to upvote him no problem.
Just thought you might like to know :-)