Can no longer install/update plugins through admin iface

Hello everybody! I’ve had WP on my website for quite a while now, and never had a problem with it until recently. Since the update to 2.9 (installed through the admin interface because I’m too lazy to do it manually), I have been unable to install or update plugins via the interface. It downloads the archive into the wp-content directory just fine, but stops when it hits “Unpacking the package.” There isn’t any delay at all from the time that is flushed to the browser from the server, and it appears that the execution of the scripts stops immediately after echo’ing “Unpacking the package.” and flushing. Doing error_reporting(E_ALL) displays no error messages, only a few warnings about some plugins redefining constants.

Any thoughts or ideas? Like I said before, I’ve had this WP install running flawlessly for well over 6 months on this particular hosting account, and for years longer (but with different hosts). Thanks in advance for the help!