I tried this plugin with a 1.4.1 installation with no problem but now after a fresh install of 1.4.2 I cannot enable that, if I chose it and then apply I got this message:

Catchable fatal error: Object of class __PHP_Incomplete_Class could not be converted to string in *****zp-core/zp-extensions/user_login-out.php on line 56

for quick reference that line contains:
$candidate = array_unique($candidate);

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.