Could this be the same problem as described in this bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=930919
There is also a patch linked that you could try.
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- Bug#942308: dovecot-core: Sieve replication does not... Daniel Botting
- Bug#942308: Further information Daniel Botting
- Bug#942308: Maybe duplicate of #930919 Maximilian Engelhardt
