I would suggest reaching out to Esri Technical Support and they will be able to help troubleshoot the issue to narrow down what's happening.
Survey123 sends a queryRelatedRecords REST request when accessing the related records in the inbox. I'm thinking the error is coming from the server side. Sending the request outside of Survey123 would be able to confirm if that's the case.
Has this issue started recently? Has the inbox query worked in the past and this behavior came up recently, or has it never worked?