Late to see this - it seems to be a typical problem with 'complex' documents that fail to be rendered within the timeout limits.
The trick is to locate all the documents that have the failed thumbnail aspect applied to them - which alfresco must do by itself when that error is generated. I have a list of 'dubious' proxy ip addresses that renders to over 5000 pages 
so I was lucky to have a document that I knew was the source of the error.
So, to the partial solution, which will at least allow you to locate the failed documents.
Go to search box at upper right, type/paste in the below and press enter:
ASPECT:"cm:failedThumbnailSource"
Et Voila!
Works in both standard search(1) or in the node browser(2)
1) just paste that into your search box in upper left and press enter
2) set search type to lucene, location to the workspace://SpacesStore (can also search the archive space store for docs that failed that are in trash), paste, go
Now you will have a list of documents that failed to render - either corrupt, "too complex", too large, take too long to render and timeout, or any of the former.
[Actually I am not 100% certain about which errors result in the failed thumbnail aspect, but certainly large and/or complex office documents and some PDF's cause problems. Actually I had a png that was causing probs too - dl -> gimp, re save, upload, fixed]
I have fixed some PDF's simply by downloading them, "printing" to a new PDF, then uploading it back. Often the size is almost identical but one renders and one does not...the vagaries of standards.
Similarly I have downloaded a doc produced under MS Office on Windows to Linux, opened in Libreoffice & saved it again as xls(x)/doc(x) (and done nothing else) and uploaded it back and it was fine and rendered perfectly.
YMMV
