Symptoms

Confluence fails to install a new plugin from the interface. This can happen when using third party plugins as well.

The following appears in the atlassian-confluence.log:

[atlassian.upm.impl.ObrPluginInstallHandler] installPlugin Failed to remove local OBR repository resources
com.atlassian.upm.PluginInstallException: Failed to install OBR jar artifact
	at com.atlassian.upm.impl.ObrPluginInstallHandler.installPlugin(ObrPluginInstallHandler.java:101)
	at com.atlassian.upm.PluginInstaller.execute(PluginInstaller.java:164)
	at com.atlassian.upm.PluginInstaller.install(PluginInstaller.java:119)
	at com.atlassian.upm.rest.resources.install.InstallTask.installFromFile(InstallTask.java:110) 

Cause

There are 2 reasons for this issue.

1) The max packet size in MySQL is set to a low value which is not allowing the plugin to be installed

2) The UPM is running an old version and must be updated to the latest one for the confluence version.

Note there might be other reasons rather than the ones above which were not found yet.

Resolution

1. Follow these steps to setup a higher value to the MySQL max packet size.
2. Upgrade the UPM to the latest available version

Restart the application and try to install the plugin again.