Please check the file __css.php that you can find in epesi\data\Base_Theme\templates\default\ folder when the error occurs. If the file is not there, run update to default theme, load epesi and check for this file again. Make note of current file permissions. Depending on the hosting you have setup epesi on, there might be some restrictions as to how php files with full access (777) are handled. If that's the case, manually change the permission and this should fix the problem.
Hope this helps,
Arek