I had a look into the error log. There where a lot of this messages:
[Wed May 15 14:07:18 2013] [error] [client 141.5.24.1] client denied by server configuration: /web/rgred/www/rg2/data/security.png, referer:
http://rg2.wiki.gwdg.de/doku.php?id=rg:start&do=admin
The security.png file is an image that says "It seems that your data directory is not properly secured" The Information I got with my wiki is that the document root to the wiki is www/rg2 while the webpath to document root is /web/rgred/www/rg
I do not know much about web server but maybe the problem is, that the plug-in installation tries to create a path begining with /web/rgred/... and is thus identified like an external user trying to break into the system? If so, would it be solved if the installation script starts at the document root point www/rg2 ?
The error log shows for one of the failed installation intents this I think:
[Wed May 15 14:42:41 2013] [error] [client 141.5.24.1] client denied by server configuration: /web/rgred/www/rg2/data/security.png, referer:
http://rg2.wiki.gwdg.de/doku.php?id=rg:start&do=admin
[Wed May 15 14:42:41 2013] [error] [client 141.5.24.1] client denied by server configuration: /web/rgred/www/rg2/data/security.png, referer:
http://rg2.wiki.gwdg.de/doku.php?id=rg:start&do=admin
[Wed May 15 14:57:34 2013] [error] [client 141.5.24.1] PHP Warning: file_exists(): open_basedir restriction in effect. File(/web/rgred/www/rg2/lib/plugins/ckgedit/fckeditor/userfiles/file/) is not within the allowed path(s): (/web/rgred/www/rg2:/tmp) in /web/rgred/www/rg2/lib/plugins/ckgedit/fckeditor/editor/filemanager/connectors/php/io.php on line 114, referer:
http://rg2.wiki.gwdg.de//lib/plugins/ckgedit/fckeditor/editor/filemanager/browser/default/browser.html?Type=File&Connector=http://rg2.wiki.gwdg.de//lib/plugins/ckgedit/fckeditor/editor/filemanager/connectors/php/connector.php?&CKEditor=wiki__text&CKEditorFuncNum=3&langCode=de
[Wed May 15 14:59:04 2013] [error] [client 141.5.24.1] PHP Warning: file_exists(): open_basedir restriction in effect. File(/web/rgred/www/rg2/lib/plugins/ckgedit/fckeditor/userfiles/image/) is not within the allowed path(s): (/web/rgred/www/rg2:/tmp) in /web/rgred/www/rg2/lib/plugins/ckgedit/fckeditor/editor/filemanager/connectors/php/io.php on line 114, referer:
http://rg2.wiki.gwdg.de//lib/plugins/ckgedit/fckeditor/editor/filemanager/browser/default/browser.html?Type=Image&Connector=http://rg2.wiki.gwdg.de//lib/plugins/ckgedit/fckeditor/editor/filemanager/connectors/php/connector.php?&CKEditor=wiki__text&CKEditorFuncNum=3&langCode=de
Just thinking loud while waiting for an answer from the system manager...