The code in ssl_protocols_to_min_protocol() to convert ssl_protocols to min/max values can't cope with strings containing "!SSLv2".

dovecot: imap-login: Fatal: Unknown ssl_protocols setting: Unrecognized protocol 'SSLv2'

This string might be configured explicitly by the user, or if the user hasn't configured this themselves it could also come from the default because master_service_ssl_default_settings sets this:

#ifdef SSL_TXT_SSLV2 .ssl_protocols = "!SSLv2 !SSLv3", #else .ssl_protocols = "!SSLv3", #endif