Bad server version ProtocolErrorCode=209

As a bit of background: I have been working through a number of issues with my app which already uses realm sync (realmSwift 10.16.0). The issues, which mainly revolved around having an expired login token whereby users were not automatically logged out (along with destructive changes to my schema), have lead me to terminating sync, then reinstating the sync. I believe I have addressed these problems through testing in a test environment (which included upgrading to realmSwift 10.26.0), however, when I push these changes to my production environment I am now seeing the following error. The error occurs upon users re-logging in to the app (due to the expired token).

client file contained lastIntegratedServerVersion(8) greater than the downloadServerVersion(3) sent in the IDENT message which is not allowed (ProtocolErrorCode=209)

I am happy to troubleshoot myself, however, I don’t really know where to start with the above error, as the only mention I have been able to find of ProtocolErrorCode=209 is in: Integrating changesets failed: error finding the previous history size for version 80: client is disconnected (ProtocolErrorCode=101) - #4 by Mansoor_Omar.

I appreciate the suggestion in the above link to open a ticket, however, as I said, Im happy to debug this one myself with a few clues. Also, I should mention, I expect to be running in to a BadClientFileIdent error, which I believe I am already handling properly as per my test environment, as opposed to the ProtocolErrorCode=209.