It is not the respondent, who should have the interviewer account. It is the responsible for the assignment, who must be in the interviewer role (in other words the assignment must be assigned to an interviewer). You don’t have to create interviewer accounts for your respondents to collect CAWI surveys (though it’s possible in principle).
If the above is just weird wording, but you believe the setup is correct, contact the support directly with
the server address,
assignment number and link, at which the above error was obtained,
date/time/ip when the URL was accessed,
the browser descriptives.
Since this is an “unknown error” there is nothing that can be said just from this message alone.
PS: make sure this is repetitive, not a temporary unavailability due to e.g. today’s release of v20.01.