PGP Desktop Email 10.2, Outlook 2007, Exchange 2007 won't encrypt
I have deployed PGP Desktop Email 10.2 on 8 identical PCs in a Microsoft SBS 2008 network. All are running Windows 7 32bit and Outlook 2007. On 5 of them it works fine.
On the remaining 3, it won't encrypt or decrypt anything. If I write "[PGP]" in the subject line or check the "encrypt" option, the mail arrives at the destination unencrypted. If an encrypted mail arrives, it shows up in Outlook as empty, with an attachment named "encrypted.asc" which Outlook cannot display.
I have double and triple checked the configurations between working and non-working PCs and they are, to my eye, identical. The PGP Desktop log shows "processing" lines for all outgoing and incoming mails, even displaying the subject line containing "[PGP]", but doesn't actually process them. The only suspicious message is
16:06:58 E-Mail-Adresse Fehler Aufgrund von unzureichenden Berechtigungen ist die Proxyfunktion für den Vorgang 2372 nicht möglich. Beim Vorgang handelt es sich möglicherweise um einen Dienst.
(E-Mail Address Error Because of insufficient permissions the proxy function for process 2372 is not possible. The process may be a service.)
which appears once after PGP Desktop is started, and sporadically during the day, but not in direct connection with sent or received mails, whether encrypted or not.
The Windows event log is clean.
What can I do to find and ideally correct the cause of that malfunction?