SUGGEST/QSUGGEST lookup over the dictionary index, right now they only operate on a single index. Maybe add a feature request ( here or make a github ticket) for distributed index support (just like CALL KEYWORDS has now).
Regarding your error, are you sure you connect to Manticore 2.5.1? Maybe you are still connecting to an older version of Sphinx. When 'mysql -Pport -hhost" what does the welcome string says? Does CALL QSUGGEST works?
I was running 2 instances in some way, following the upgrade to Manticore I still had a few things to fix,
The funny thing was that the welcome string was correct and returned I was running 2.5.1