I just reinstalled my Macbook. After having restored my applications I launched OG and found there was an upgrade available. When trying to upgrade the application I ended up with an error message: “couldn’t communicate with a helper application”. The same happens with OmniWeb.
Update: after I had performed an OmniWeb upgrade by hand (downloaded a new file from OmniStaging) I was again able to upgrade to the next snapshot build by hand.
As the error I encountered with OmniWeb was exactly the same as with OmniGraffle I suspect that the reason is the same, too. This seems to indicate that OmniSoftwareUpdate somehow did not like my installations which I had recovered from backups.
I therefore reinstalled OmniGraffle and will have to see if I will be able to upgrade it next time an upgrade is available.