Accidental gap - Glottopedia
There's no accidental gap for 'BIK'. One could say that 'unsad' is one of these words, since 'unhappy' exists. But 'sad' is the marked antonym, so it would be less likely to have a modifier like 'un'.
Yes, I think 'argumentation' might have been one - but I haven't done the research on the history of the word.
The following words do exist: documentation, commentation, so prima facie, 'argumentation' was possible, and a gap.
If it is, then the following are also accidental gaps: *agreementation, *developmentation ...