Created Unassigned: Certificate is invalid, although it is not [270]

Dear WPP-Team,

we get a message on one off our WSUS maschines with WPP installed, which says the "Certificate is invalid" and we will be not able to publish updates. The certifcate indeed is valid (till 2016/1/2) since we use the same on 2 other WSUS maschines with WPP. If we try to load the certificate again, we get the message "Failed to load the certificate". Check the file. The load and import on another WPP and into Windows was successfully, so it seems not to be the certificate itself. Any thoughts or ideas?

Environment is:

OS = Windows Server 2012 R2 Standard
WSUS = 6.3.9600.16384
WPP = 1.3.1504.15
WSUS and WPP on same maschine.

I've attached the version screenshot for you.

Thanks in advance
-Andreas-