Adding lda_twostep_ruleproc=1 and lda_mrautoproc=yes fixes the issue on all of my servers.
However, this was quite unexpected from an administrator's perspective. After the update, inbox rules simply stopped working, while both Outlook and grommunio Web continued to show them as enabled and correctly configured.
Since nothing indicated that the rule processor configuration had become relevant, troubleshooting this took quite some time. On an existing installation, one wouldn't normally expect inbox rules to stop working after a routine update due to a missing configuration option.
