The plugin has not changed since 1.4, it is the same file in 1.4.0.1[sic] and 1.4.0.2[sic]. It will change in 1.4.1, though, and who knows about 1.4.2.
There seems no good reason for that error. the `$candidate` variable can be nothing but an array. Also, the `array_unique` function should not be desiring a string, therefore PHP should not be attempting to convert `$candidate` to a string.
Everything points to something having been corrupted. I suggest you place a `var_dump($candidate);exit();` statement just before that line to see wht `$candidate` is.