When I install any extension, my site goes into maintenance mode. apache logs are empty.

[kireol]

When I try to install any extension, my site goes offline (maintenance mode).

I'm installing them through the magento connect manager.

An example extension would be http://connect20.magentocommerce.com/community/TBT_Enhancedgrid

I have some extensions previously installed.

I'm running  Magento CE ver. 1.9.0.1 on an ubuntu LAMP machine.

I've tried removing any unused modules and clearing var -> cache

What should I do?

[kireol]

Still broken.  Any ideas?

[brakoops]

use your FTP client to remove the file maintenance.flag. When installing the extension again, disable the option 'Put store on the maintenance mode while installing/upgrading/backup creation' first