Activation Issue

I’m having an activation problem that doesn’t seem to be related to the other issues I’ve found. When I go to activate, I get this error:

Parse error: syntax error, unexpected ‘[’ in anspress-question-answer/anspress-question-answer.php on line 569

I’m on PHP 5.5, and WordPress 4.8.1, AnsPress 4.0.5. No Theme, no other plug-ins. Any help is greatly appreciated!