Same problem for me - all of a sudden my iPhone calendar was showing all day entries from 1st Nov to 27th Dec for this; unable to decline invitation or delete etc.
After going thru settings > Calendar > accounts, noticed that my outlook account (for which I use for email) was also set (via the green button) to sync outlook calendar. I deselected this option (but left email still active) & okayed warning from iPhone that it would delete the outlook calendar entries and lo & behold the suspicious entries have now disappeared from iPhone calendar! Never use outlook calendar anyway - only always the iPhone calendar so no loss not to synch with outlook. And it certainly seems to have sorted issue. Looks like problem is with outlook….