Symptoms

2010-02-04 14:13:09,685 ERROR [http-443-4] [atlassian.plugin.loaders.ScanningPluginLoader] deployPluginFromUnit Unable to deploy plugin 'null', file Unit: D:\confluence\data\plugin-cache\1265310789404upload_14bb31bb_1269912ae5d_7f90_00000008.tmp (1265310789404)
– referer:

| url: /admin/plugin-repository/upload.action | userName: Patrick Gisbey | action: upload
java.lang.IllegalArgumentException: The artifact URI

is not a valid plugin artifact
at com.atlassian.plugin.DefaultPluginArtifactFactory.create(DefaultPluginArtifactFactory.java:38)
at com.atlassian.plugin.loaders.ScanningPluginLoader.deployPluginFromUnit(ScanningPluginLoader.java:112)
at com.atlassian.plugin.loaders.ScanningPluginLoader.addFoundPlugins(ScanningPluginLoader.java:176)
at com.atlassian.confluence.plugin.DatabaseClassLoadingPluginLoader.addFoundPlugins(DatabaseClassLoadingPluginLoader.java:53)

Cause

This is a known bug in the plugin repository client, which by itself is a plugin. Refer to APR-90 for more information.

Resolution

1. Remove this plugin from the database:

   select plugindataid,pluginkey,filename,lastmoddate from PLUGINDATA where filename like '%.tmp';
   delete from PLUGINDATA where plugindataid='<plugindataid>';
   

   <plugindataid> is the plugindataid value returned by the select query.

2. Clear the plugin cache - see How to clear Confluence plugins cache
3. Start Confluence and re-install the add-on using the Universal Plugin Manager.  See Installing Marketplace apps for how to do this. 

See also How to manually remove malfunctioning add-ons.