IPN log error (I believe)

My hosting company have informed me of this error in the log:

[Fri Jan 18 10:23:24 2013] [error] [client 173.0.81.1] PHP Warning: fwrite()
expects parameter 1 to be resource, boolean given in
/var/www/svia575095/htdocs/wp-content/plugins/paid-memberships-pro/services/ipnhandler.php
on line 334
[Fri Jan 18 10:23:24 2013] [error] [client 173.0.81.1] PHP Warning: fclose()
expects parameter 1 to be resource, boolean given in
/var/www/svia575095/htdocs/wp-content/plugins/paid-memberships-pro/services/ipnhandler.php
on line 335

I have looked at the code and it looks like it’s writing to a logfile and then closing it.

I’ve found a logfile (log.txt) in wp-content/plugins/paid-membership-pro/logs but there’s nothing in it (zero bytes). Is this the file concerned, and is it a file permissions problem?

https://www.ads-software.com/extend/plugins/paid-memberships-pro/