Test procedure:

1. Install piwik-2.15.0-b8 in a subdirectory (e.g. /p) on a remote server (see installation issue on empty database: Unknown table engine 'InnoDB' #8824 for DB access issues on installation and afterwards)
2. Log in as super-user and go to Administration -> Plugins
3. Deactivate a plugin (e.g. ExampleAPI)

Expected results:
3. The Plugins page reloads (using correct redirect) with the plugin shown as inactive

Actual results:
3. Following redirect occurs: GET http://[...]/p/index.php?module=CorePluginsAdmin&action=deactivate&pluginName=ExampleAPI&nonce=b577c83c4293bc846f732510a5ed66f7 responds with status code 302 Found and header Location: http://[...]/ (i.e. the root directory of the Piwik installation domain).

There are two issues (which may have the same root cause) with that Location header:

• The Piwik installation subdirectory (e.g. /p) is not considered - domain root is used instead
• The index.php?module=CorePluginsAdmin&action=plugins&idSite=1&period=range&date=last30, etc., part is missing

That is all on a remote server with PHP 5.5.22, MySQL 5.1.73 with PDO\MYSQL.

Notes:

• The issue does not occur on plugin activation, but only on deactivation
• piwik-2.15.0-b8 is the first I tried after piwik-2.14.3, so I don't know which beta this issue occurred with first.
• I don't know if this issue is related to the above mentioned installation issue on empty database: Unknown table engine 'InnoDB' #8824