Error in main.error.log (Undefined Index: HTTP_HOST)

Hello fellow Piwik-users

My server-admin informed me, that one of our Piwik-installation is fillin up the apache error log (main.error.log).
It is full of Messages like this one (below, I added a version without the html-stuff for clarity):

There is an error. Please report the message (Piwik 1.9.2) and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).

Notice:
Undefined index: HTTP_HOST in /var/www/stats/core/Url.php on line 188

Backtrace -->

#0 Piwik_ErrorHandler(...) called at [/var/www/stats/core/Url.php:188]
#1 Piwik_Url::isValidHost(...) called at [/var/www/stats/core/Controller.php:499]
#2 Piwik_Controller::setHostValidationVariablesView(...) called at [/var/www/stats/plugins/Login/Controller.php:87]
#3 Piwik_Login_Controller->login(...) called at [/var/www/stats/plugins/Login/Controller.php:46]
#4 Piwik_Login_Controller->index(...) called at [:]
#5 call_user_func_array(...) called at [/var/www/stats/core/FrontController.php:138]
#6 Piwik_FrontController->dispatch(...) called at [/var/www/stats/core/Controller.php:719]
#7 Piwik_Controller->redirectToIndex(...) called at [/var/www/stats/plugins/CoreHome/Controller.php:42]
#8 Piwik_CoreHome_Controller->redirectToCoreHomeIndex(...) called at [:]
#9 call_user_func_array(...) called at [/var/www/stats/core/FrontController.php:138]
#10 Piwik_FrontController->dispatch(...) called at [/var/www/stats/index.php:53]


^C

The same in human-readable form:

There is an error. Please report the message (Piwik 1.9.2)
and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).
Notice: Undefined index: HTTP_HOST in /var/www/stats/core/Url.php on line 188
Backtrace
#0 Piwik_ErrorHandler(…) called at [/var/www/stats/core/Url.php:188]
#1 Piwik_Url::isValidHost(…) called at [/var/www/stats/core/Controller.php:499]
#2 Piwik_Controller::setHostValidationVariablesView(…) called at [/var/www/stats/plugins/Login/Controller.php:87]
#3 Piwik_Login_Controller->login(…) called at [/var/www/stats/plugins/Login/Controller.php:46]
#4 Piwik_Login_Controller->index(…) called at [:]
#5 call_user_func_array(…) called at [/var/www/stats/core/FrontController.php:138]
#6 Piwik_FrontController->dispatch(…) called at [/var/www/stats/core/Controller.php:719]
#7 Piwik_Controller->redirectToIndex(…) called at [/var/www/stats/plugins/CoreHome/Controller.php:42]
#8 Piwik_CoreHome_Controller->redirectToCoreHomeIndex(…) called at [:]
#9 call_user_func_array(…) called at [/var/www/stats/core/FrontController.php:138]
#10 Piwik_FrontController->dispatch(…) called at [/var/www/stats/index.php:53]

The problem only exists on one of our Piwik installations (running 1.9.2).
I did not find any information useful to me on this particular error, neither in this forum nor at Google.
Any help or hint would be apreciated.

Best regards,
Wolf

fixed in last version

Thank you, I will do an update asap.