Cannot create file: newbb_permission_forum_access.php

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Error: Smarty error: unable to write to $cache_dir 'cache'. Be sure $cache_dir is writable by the web server user.

Backtrace:
class/smarty/Smarty.class.php (1095)
class/smarty/internals/core.write_cache_file.php (81)
class/smarty/Smarty.class.php (1281)
class/smarty/Smarty.class.php (1108)
modules/newbb/rss.php (175)