There are 2 errors on my join page

Message: file_put_contents(/var/www/vhosts/mysite.net/httpdocs/socialnetwork/ow_log/error.log): failed to open stream: No such file or directory 

File: /var/www/vhosts/mysite.net/httpdocs/socialnetwork/ow_system_plugins/base/classes/file_log_writer.php 

Line: 61 

Message: chmod(): No such file or directory 

File: /var/www/vhosts/mysite.net/httpdocs/socialnetwork/ow_system_plugins/base/classes/file_log_writer.php 

Line: 65 

There are no errors if users Sign up with social networks.  Strange :-(

Any help?