Just sharing my experience with this update. I'm aware that there have been quite a few folks with troubles as reported on the dovecot mailing list.
My own experience was
a) dovecot itself worked with no problems at all.
b) I did however have pigeonhole failing which was perplexing - however restarting dovecot a second time got the sieve working and over the last half day or so everything is working well. I have noticed for users with no sieve script there remained some fails running the default sieve script. I am recompiling that script (sievec) manually to see if that helps.
gene