ical 6.0 and Office 365... I can see my calendar but can't add or modify events

Recently my company moved fron Exchange on premises to Exchange Online (Office 365). Everything is fine with my e-mail and contacts. I configured my account from System Preferences/Mail, Contacts and Calendars where I selected the 4 options: Mail, Contacts, Canlendars & Tasks and Notes.

I did same on my iPhone and iPad with no problem at all.

When iCal (in my Mac) is started I can see all of my events in my Exchange Calendar. The point is when I try to create a new one or to modify an existing event, it's not possible and a big screen appears with the legend "iCal cannot save "XXXX" event in the Exchange Server".

Is there something I can do to fix this out, or not?

In my iOS devices I don't have this issue, only in my MacBook Pro. If anybody can help me to solve this I'd appreciate it. Thanks.

Renato