If you share the details on how you have gone about this in CH, CM and MRA systems then we can have a look and verify if there are anything missing or incorrectly done.
Are the two CM systems in an ILS network and have you verified that the end user accounts only have Home Cluster set in the CM 15 system? Also check that the UDS SRV records for the domain is correctly pointing to the CM 15 system.
Then I would recommend you to collect the logs from a Webex client, and possible the same from a Jabber client, and run them trough the analyzer at https://cway.cisco.com/csa-new/#/home to see if there are any differences between them.
There is really not that much to this. We have the exact same setup as you outlined, more or less anyway, I'll go into the differences later. In Control Hub we have two UC Manager Profiles defined.
Production, that runs on the CRS 14 platform
Test (pre-production) environment that runs on the CRS 15 platform
Even if they are blurred you can likely see they are just specifying different domains or in reality a domain and a sub domain.
What you might have missed out on is that the Webex client will use the domain in the login name, ie in the email address to do the SRV look up. It does not typically use the domain that you put into the UC Manager Profile.
What we do that might be different from you is that a user that is on the pre-production systems can actually login with just the domain name in the email address, ie he/she do not need to enter the sub domain (next.domain) as the email address when logging in to the Webex application. This is working as our systems are in a ILS network and they distribute information about home cluster of the user between each other, so even if the user initially resolves the UDs record to a CM 14 node it will get informed by that node that you belong to this CM 15 system and from there on the communication happens between that system and the client.
