Hi,
I hope you have checked this before downgrade. [url]https://dev.mysql.com/doc/refman/5.6/en/downgrading-to-previous-series.html[/url]
Generally, for this kind of error for mysql tables solved by creating mysql db from scratch. If you have old backup (of 5.5) then you can also restore those files for specific tables. Something like this [url]innodb - Cannot open table mysql/innodb_index_stats - Database Administrators Stack Exchange and check.