Just in case anyone else has users getting errors when they rather tardily try to activate a MadCap account: the invitation sent out from Central times out after "5 days or a week roughly" (I quote from the email from Support), and slowcoaches will get a message telling them the invitation can't be found. The solution is to Reinvite the user. At least, knowing this, I can warn folk to sign up quick!
Ideally, having the expiry as something an author can configure would be better. A week is not a long time and, especially over the summer, might expire whilst someone is on holiday.
User Invitations Timeout
User Invitations Timeout
Rae Streets (Flare user since 2017, now on Flare 2023; Central user from 2020)