Here are more details :
-N9 is OK to sync google calendars using CalDAV
-The problematic CalDAV account was OK before some changes on my MFE account (and some issues with /home that required a eMMC flash)
-I done a capture of network activity when syncing this problematic calendar with ThunderBird (OK) and N9 (KO), I can't see many differences. It seems that N9 tries to get some contents that are not available (global HTTP code is 207, inside this, severals are 404 and 200).

So I understand that the N9 kept a track of the defective calendar and is trying - without success - to get some specific informations.

Hence, my question is now simple : how to 'fully' reset whole N9 calendars ?