MacOS – iCal compatibility on Mountain Lion

I just upgraded a MacBook Pro running Snow Leopard to Mountain Lion.

On Snow Leopard I was running iCal and had 10 years of history inside.

Now on Mountain Lion, this application was renamed Calendar, and I find all my 9 calendar names but they are empty.

I just get all the Birthdays and 2 subscribed calendars filled.

Looking at /var/log/system.log I found 2 hours of error messages like:

Sep  2 18:41:01 localhost mdworker[759]: UTF8 encoding failed for Ãloïse @ Juterbog
Sep  2 18:41:02 localhost mdworker[759]: Date validation error: EXDATE = ' 20020916T220000Z'
Sep  2 18:41:02 localhost mdworker[759]: Date validation error: EXDATE = ' 20021022T220000Z'
Sep  2 18:41:02 localhost mdworker[759]: Date validation error: EXDATE = ' 20020620T220000Z'
Sep  2 18:41:02 localhost mdworker[759]: Date validation error: EXDATE = ' 20010414T220000Z 20010415T220000Z'
Sep  2 18:41:02 localhost mdworker[759]: UTF8 encoding failed for vacances scolaires été
[...]
Sep  2 18:41:02 localhost mdworker[759]: Invalid char : (58) for mParamName in line 432
Sep  2 18:41:02 localhost mdworker[759]: Invalid char . (46) for mParamName in line 432
Sep  2 18:41:02 localhost mdworker[759]: Invalid char   (32) for mParamName in line 432
Sep  2 18:41:02 localhost mdworker[759]: *** WARNING Invalid component, discarding: Error Domain=com.apple.iCalendar Code=1000 "DTSTART is required for VEVENT." UserInfo=0x7fada22d24a0 {NSLocalizedDescription=DTSTART is required for VEVENT.}

These error messages look like they are clearly related with the migration failure of my calendars.
Unfortunately no explanation no error message was displayed to explain how to fix this.

What is the way to recover all my iCal personnal calendars?