The way commit sync works today is that it does its thing on the master RE such as expanding the groups, propagating foreing files, 'asking' each daemon if they're fine with the new config etc. Once everything turns out to be fine, the candidate configuration file is then sent to the master RE, so it can do the same.
In your case, since it looks like there's been a communication problem with the backup RE, and assuming this did/does not happen every time, I would suggest to look at some log files, mainly for any clues:
- anything else you might have, that captures 'any' level.
If that's a frequent thing, commit check | display detail might be worth looking into, too.
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