[Plugin: Simply Poll] (Error) Warning: fopen…

Hi there, this looks like an awesome plugin but when I install it i get the following message:

Warning: fopen(/home/tran56/public_html/wp-content/plugins/simply-poll/log) [function.fopen]: failed to open stream: Permission denied in /home/tran56/public_html/wp-content/plugins/simply-poll/lib/logger.php on line 27

Then my website gets all messed up and I have to deactivate the plugin. Any idea what causes this? Thanks.

http://ww.wp.xz.cn/extend/plugins/simply-poll/