Try logging to your server through FTP and try to locate and if they exist delete the following directories:
/administrator/components/com_sef
/components/com_sef
/plugins/content/joomsef
/plugins/system/joomsef
/plugins/system/joomsefgoogle
/plugins/system/joomseflang

Then try installing JoomSEF again.

I have the same error message:


this message was followed by a second message:


below both of these messages displayed the standard message about the successful install and all the htaccess contents.

joomsef was my first component installed after a fresh install of joomla 2.5.1.

Hi,
Could you post some more information about your system? Specifically:
Is your web server Apache, IIS or something else?
What version of PHP is it running?
What web browser and its version did you use when installing JoomSEF and on what OS (Windows/Mac/Unix)?

Thank you

Dear Dajo:

Log in FTP ,there is no any directories you listed here,

and you can find my system information in the attachment.

I am using window7

can't add file ,so i copy it here:
系统信息
设置 值

服务器信息 Linux cat112.redphp.cn 3.0.18-x86_64-linode24 #1 SMP Mon Jan 30 14:25:46 EST 2012 x86_64
数据库版本 5.1.41-3ubuntu12.10
数据库连接校对 utf8_general_ci
PHP 版本 5.3.2-1ubuntu4.14
Web服务器 Apache/2
Web服务器到PHP的接口 apache2handler
Joomla! 版本 Joomla! 2.5.1 Stable [ Ember ] 2-Feb-2012 14:00 GMT
Joomla!框架版本 Joomla Platform 11.4.0 Stable [ Brian Kernighan ] 03-Jan-2012 00:00 GMT
User Agent Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.2 (KHTML, like Gecko) Chrome/15.0.874.120 Safari/535.2

There may be some incompatibility with PHP 5.3.2. We'll test it and let you know. Thank you for the informations.

Hi,
We were unable to replicate the problem with both free and paid versions of JoomSEF. Tested on Joomla 2.5.1 and 2.5.2 running on PHP 5.2.9 and PHP 5.3.2.
The error message is displayed when the component is not correctly stored to database. Try disabling cache in Joomla if it's enabled and install JoomSEF again. If it doesn't help, check your database configuration and whether the _extensions table is not corrupted.
We can also try finding the problem directly on your server as a paid service: www.artio.net/de/e-shop/support-services...a-related-support-1h