In the Client trace we see the below error message:
26/09/2019 16:53:04.201[9208][L]: EVMSP:
26/09/2019 16:53:04.202[9208][M]: EVMSP:
26/09/2019 16:53:04.202[9208][L]: EVMSP:
This happens because Virtual Vault is set to read only because attribute VV_ForceReadOnly is set to True and this attribute is stamped in hidden message.
In order to check the VV_ForceReadOnly attribute access the Vault Information from the Client. In order to access Vault Information press CTRL + SHIFT + Left Click on any Enterprise Vault button on Client.
Click on Vault Information shown in the below screen:

Delete the hidden message from the mailbox and synchronize the mailbox which will add the hidden message again in the mailbox.
In order to delete the hidden message refer to this article:
How to delete hidden messages from Outlook on the client in Enterprise Vault
https://www.veritas.com/content/support/en_US/article.100018108.html