This needs some debugging to work put whats going on. The message is deliberately vague to prevent bots and non authorised participants.
Firstly lets check the obvious one. Is the survey open. Try the live preview on the activity screen. Can you see the survey or do you get the same problem. If so it‘s likely you have hit an overall quota (the undefined segment) or the end date has lapsed.
if you can see it then it could be the panel linking is sending them to a different survey or the parameters have been screwed up, a common problem is using the url from the browser, rather than the invitation link
Finally, the timeout default of 30 minutes may be too short,
Please share what you‘ve found and we‘ll solve it together