Downloaded today from http://www.myplugwise.com
Fortunately the installation created a system-restore point which, for the first time in my life, helped me.
After the system-restore I only had to copy the old database .mdb and the old release is now running again..
(I backup the plugwise database .mdb every day

Still would love to know why this happened and if it can be fixed. (even though I'm not to sure if I need to upgrade reading through some of forum messages)