As of today, the netmirror.org server seems to be having a hiccup. The serendipity Plugin Spartacus by default requires this server to provide automatic plugin and theme updates and downloads. The inavailibility of this server can lead to timeouts and not being able to install new plugins.
The issue should resolve itself once netmirror.org is up again. Temporarily you can either reconfigure your Spartacus-plugin to use the SourceForge-Mirrors, or completely disable the Spartacus plugin. Using the s9y.org mirror will not help you, as this server is currently not hosting any files.
I will post an update once the server is up again. Sorry for the inconvenience.