EffluvesLiquides

Ok, Maybe you have access to your server error log. Have a look at it, you may find the answer. Whatever, there's certainly a problem with your Smarty cache. You have to find what changes you did which caused this error. I don't know exactly how works your 1.4 version since I'm a 1.5 beginner. You have to clear your cache, it will surely fix this issue.

Yes, that's it, for sure. Did you made a backup of the header.tpl ? If yes, just upload the old (fonctionnal) file. If not, you have to be sure to revert your changes. The file MUST be as it was before you tweaked it.

Mhhh, I'm not sure. I run a french 1.5.6.2 version, and it's located in 'parametres avances'->'performances' There's a button in the Smarty area 'Effacer le chache de Smarty et le cache de l'autoload', wich I could translate by 'Clear Smarty cache and Autoload cache'. Sorry for my bad english (not my native language)...

Hello, Try to clear your Smarty cache.