I have Concise Oxford English Dictionary bought from Mobipocket on my Cybook. When looking up "fastidious" if says "No result" but the word
is in the dictionary and I can look it up with the same dictionary
using my Palm T5. The word "fast" gives the alternatives "FAA", "FAS",
"fat" and "faze" which seems totally wrong.

I sent a bug report to Bookeen but was curious if this happens for other people also and if it happens with other dictionaries?

Just checked with my Chambers dictionary and it's fine there. Sounds like a problem with the indexing of the dictionary.

I got an answer from Bookeen that gave an estimated time for a new firmware release: "We are going to debug this issue (has been added to our database). It should be fixed in the next software update (which we is scheduled to be released December 17-28)."