Having the same problems on my MacPro. I can open my events editing window and edit everything in the event except the title of the event. I have also done everything suggested on this link, to no avail. Even tried trashing all my calendars and creating a new one. Does the same. Yet the same calendar is running on my Macbook with no problem. Could this be an issue unrelated to iCal. Maybe a permissions issue?
I have this problem since Yosemite. No suggestion helped. No deleting, restarting, ... helped. Some times I can edit, sometimes not. Frustrating! I have found out, that the iCal Edit windows appear but in a region of the screen I cannot reach. My workaround is to: 1) mark the event 2) go to one-day-view 3) edit the event there 4) go back to the desired view. Is it really Apple like to develop "workarounds"?