I received the below two errors when viewing WP-Forum after an automatic upgrade via the WordPress admin plugin page.

Warning: file(<wp-root>wp-content/plugins/wp-forum/wp-forum.php) [function.file]: failed to open stream: No such file or directory in /home/question/www/www/wp-content/plugins/wpforum/wp-forum.php on line 33

Warning: implode() [function.implode]: Invalid arguments passed in <wp-root>wp-content/plugins/wpforum/wp-forum.php on line 33

After digging around for a moment I noticed that the automatic upgrade renamed the WP-Forum directory in wp-content/plugins from wp-forum to wpforum. After renaming the directory you have to go in and activate the plugin again. This resolved my issue with PHP errors being displayed in the forum.

If you are still seeing the errors after upgrading make sure you have WP Super Cache disabled.

The instructions for this plugin state to rename the wpforum directory to wp-forum so I just think the forum was not made to auto upgrade via the wp-admin interface.

Another item to note is the latest version of WordPress has a bug with the PHP ul function as noted by Michael Visser. This causes the following error to show up:

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After installing the wp-forum plugin you may notice that the layout looks out of wack. This could be for numerous issues but one I came across was the fact that after unpacking the plugin on the server the main directory permissions were incorrect. If you chown 775 wp-forum in <web-root>/wp-content/wp-plugins/wp-forum this should resolve your issue.