The new version of Illustrator is not allowing to use the "select by:" feature, it appears blocked.

Reported as a bug at UserVoice:
http://illustrator.uservoice.com/forums/601447/suggestions/50958215

I have noticed other users reporting this as an issue in 30.1. Seems to be resolved by a prefs reset:

Issue with “select same” [v 30.1] : r/AdobeIllustrator

is “Select by” a third party option? I don’t remember seeing that option, can you show a screenshot?