I had a similar problem when trying to add a product:
I would get the message "A server error occurred while loading the tabs: some tabs could not be loaded." and the PHP log file on the server would say "Smarty: Invalid compiled template for 'header.tpl'".
I figured it had something to do with the way template files are compiled for caching and therefore tried the following:
1. I went to Advanced Parameters > Performance
2. I set Template Compilation to "Recompile templates if the files have been updated". (Previously, it was set to "Force compilation".)
This worked for me (though I'm not sure why exactly).
I should perhaps also note that other settings I use are:
Smarty - Cache = YES
Smarty - Clear cache = Clear cache everytime something has been modified
Smart cache for CSS = NO
Use Cache = NO
Thomas