Can't access Back Office

[japanknife]

Hello

 

I have a online shop with prestashop 1.6.1, the front office runs well but I have a lot of problems to access to the back office.

 

Last week I can't access to the back office,  I received an "Internal Server Error",  with the server administrator we found this error:

 

 

 

mod_fcgid: stderr: PHP Notice: Undefined index: lang_iso in /var/www/vhosts/xxxxxxx.com/tools/smarty/sysplugins/smarty_internal_templatebase.php (157) : eval ()'d code on line 88

Trying to get property of non-object in /var/www/vhosts/xxxxxxx.com/tools/smarty/sysplugins/smarty_internal_templatebase.php (157) : eval ()'d code on line 88

 

 

At  the end and I didn't know why, on Tuesday I can access to the back office.

 

 

Today I tried again and same problem " Internal Server Error " and I can't access.

 

I take a look on the server and there is the following message:

 

 

Code 500:

GET index.php?controller=AdminDashboard&token=3d84b9e7bdf0e97da21e36d79d47e8cb HTTP/1.0

 

(104)Connection reset by peer: mod_fcgid: error reading data from FastCGI server, referer: http://www.xxxx.com/index.php?controller=AdminLogin&token=387cf0f9bb05b76cfe8fcc8395083e4b

 

Premature end of script headers: index.php, referer: http://www.com/index.php?controller=AdminLogin&token=387cf0f9bb05b76cfe8fcc8395083e4b

 

Any idea?  It is a problem of Prestashop or in the server? 

 

The server administrator said that he can not help me if the problem is in Prestashop, but I think it is in the server.

 

 

Thanks in advance.