When you try to send a secured eMail, SEP-client informs
"Unable to setup or modify a secure communication channel with..." to setup or modify a secure communication channel with this user.
Your SEP-admin are not allowing you to create shared secrets by policy and your SEP-client has lost connection to the SEP-server.
How to solve it:
Go to your SEP-client and synchronize it with SEP-server. In systray, right-click on SEP-client, settings:
(SEP-client settings will open), select Profile Management tab abd notice that "Sync.Status is failed" and then click on Synchronize. Make sure that the sync status is stating: "Success" as in the picture:
Send your Secured eMail again.