Comment 6 for bug 682118

Thank for confirming that thumbnails are not generated if missing, that is basically what this entire bug is about.
I would suggest that thumbnail creation is triggered right where the missing thumbnail error is generated, on demand. Anything at startup should be avoided to prevent a slow startup.

Yes, as I wrote the log is from my current .shotwell folder since that is where the problem occured. Running fresh as you suggested worked without any problem/thumbnails got created.

Removing a thumbnail folder is usually considered a safe action. When I first asked about this issue on askubuntu, http://askubuntu.com/questions/15020/thumbnails-in-shotwell-are-all-gray that is exactly the advice I got.