Did you try using a test user or your account? I am asking because you are a moderator, so when I create a chat with the moderators group, I am added to the chat along with the moderators. When you create one, the number is one less because you aren’t added twice.
I have seen the error you saw before, too. I think it was in December when there were fewer moderators. The screenshot says 19/20, and back then, bots were still included in the count.
So far, I have not been able to reproduce the “user is invalid” error elsewhere. But the issue where the count shows 4/5 members, and the user isn’t able to create the chat, also happens on my forum.