Joomla 3 cannot disable custom plugin - plugins

Question

I just installed a custom plugin in Joomla 3 and now I cannot disable it.
When I click disable (or edit) I receive "Internal Server Error".
xml file:
<?xml version="1.0" encoding="utf-8"?>
<extension version="3.0" type="plugin" group="content">
<name>Content - Availability Calendar</name>
<author>Joomla! Project</author>
<creationDate>2010</creationDate>
<copyright>Copyright (C) 2005 - 2010 Open Source Matters. All rights reserved.</copyright>
<license>http://www.gnu.org/licenses/gpl-2.0.html GNU/GPL</license>
<authorEmail>support#msc.gr</authorEmail>
<authorUrl>www.joomla.org</authorUrl>
<version>1.5</version>
<description></description>
<files>
<filename plugin="calendar">calendar.php</filename>
<filename>index.html</filename>
</files>
<params>
<param name="unit_id" type="text" size="5" default="50" label="Villa ID" description="Villa ID"/>
</params>
</extension>

Answer 1

You're XML code is old and is based on the code used for Joomla 1.5. You need to replace the following:
<params>
<param name="unit_id" type="text" size="5" default="50" label="Villa ID" description="Villa ID"/>
</params>
with this:
<config>
<fields name="params">
<fieldset name="basic">
<field name="unit_id" type="text" size="5" default="50" label="Villa ID" description="Villa ID"/>
</fieldset>
</fields>
</config>
Then for each new parameter you want to add, simply add a new <field>