The Issue has bin solved.
the following changes have bin implemented :
change the number of treads on the proxy from 64 to 32. Here is how to modify the proxy: https://helpcenter.veeam.com/docs/vbo36 ... tml?ver=15
Proxy ID has not been properly reformed during Veeam Backup for Office 365 upgrade from version 1.0 to 1.5https://www.veeam.com/kb2364
As you can see the issue is coming from the proxy's credentials. Please, under Specify credentials to connect to the proxy server specified a local admin account instead of Jeugdhulp\BCK01$
Refer to this article if needed: https://helpcenter.veeam.com/docs/vbo36 ... tml?ver=15
The Backup is working fine now and all mailboxes are being backupped again.
thanks for all the support
With kind regards