Relation name issue with getRelatedRecords

Relation name issue with getRelatedRecords

I'm puzzled because of the following.

I have custom modules Participants (CustomModule5) and Departures (CustomModule1): a Contact can be a Participant in multiple Departures.
Participants used to be called Passengers; it was renamed to Participants more than a year ago. 

One function I have contains this code:

relParticipants = zoho.crm.getRelatedRecords("Passengers","Departures",DepartureID);
info relParticipants;

Note that the old custom module name is still used as the <relation_name>, but it functions OK: the info statment provides meaningful info about the Participants related to a Departure.

Here's my problem:
In a new function I'm writing, I want to process all Participant records related to a Contact. The info statement below:

relParticipants = zoho.crm.getRelatedRecords("Participants","Contacts",Cid);
info relParticipants;

...leads to this error message:
{"code":"INVALID_DATA","details":{},"message":"the relation name given seems to be invalid","status":"error"}

Replacing the  <relation_name> with Passengers or CustomModule5 leads to exactly the same error message.

Does anyone have any clue what I'm doing wrong?

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