can't set "Mailfilter syntax" to Maildrop

[mangoo]

I'm trying to set "Mailfilter syntax" to Maildrop (in System -> Server Config -> My server -> Mail).

It was set to Sieve; I've set it to Maildrop, but still, any filter I add is added to the .sieve file (and, an error in the .sieve.log complaining about unexpected characters).

How do I change the filtering syntax to Maildrop? If it matters, I'm using Dovecot as the POP3 server.