Sync problem Windows Phone 8.1

Hello all,

After a couple of days, synced account under WP 8.1 not working anymore.

Two causes :

1. Sometimes, it's an email blocking sync. When we delete mail, sync works one more time and... stop
2. Sometimes, it's an event on a calendar. When we delete it, nothing changes

In all cases, deleting the account from WP 8.1 and recreating it works. Even if we delete sessions from admin and recreate account on the devices, the problem persists after a couple of days of perfect working.

Shared calendars of "blocked" account seems not syncing too.

With zxAddAccountLogger, nothing appears (info level), events & mails seems well sent to device.

On the same device, Office 365 account have never failed like that.

How can i solve the problem ? Or at least debug it ?

Thanks in advance !