Hi, that message is usually generated when a PHP warning is issued. The warning may or may not actually indicate a problem in the plugin’s functionality.
It’s too bad WordPress doesn’t show the actual warning, but you might be able to find it in the PHP error log on your server.
The current version is 2.59, so as a first step I’d suggest upgrading (2.29 was developed under WordPress 3.2). I’ve tested 2.59 on my own multisite install and I don’t receive any warnings on activation. But it’s always possible there’s an issue I haven’t seen, or something specific to your server. If you have problems after upgrading, feel free to send a login and I’ll try to capture the WP message.