Hi @anonymous3542

I think this is either, not-complete updating issue or an issue with customized template files. Please delete wpforo folder in /wp-content/plugins/ directory via FTP and install it again, if you see the same error, then try to backup and remove the customized template files in WordPress active theme folder.

Also please find some error.log file to see the exact error. I believe this is not core issue, nor bug.

@robert I'm not sure. I still get "Plugin could not be activated because it triggered a fatal error." in the dashboard. In the error_log I see 

[10-Jul-2018 20:01:44 UTC] PHP Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 20480 bytes) in /home/phenomlab/public_html/wp-includes/wp-db.php on line 1889
[10-Jul-2018 20:01:57 UTC] PHP Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 20480 bytes) in /home/phenomlab/public_html/wp-includes/wp-db.php on line 1889

Thought it might be due to a custom function here

#8 / in /home/phenomlab/public_html/wp-content/themes/phenomlab/functions.php on line 166
[10-Jul-2018 20:31:03 UTC] PHP Fatal error: Uncaught Error: Call to undefined function WPF() in /home/phenomlab/public_html/wp-content/themes/phenomlab/functions.php:166
Stack trace:
#0 /home/phenomlab/public_html/wp-includes/shortcodes.php(319): vComp1('', '', 'wpforo-breadcru...')
#1 [internal function]: do_shortcode_tag(Array)
#2 /home/phenomlab/public_html/wp-includes/shortcodes.php(197): preg_replace_callback('/\\[(\\[?)(wpforo...', 'do_shortcode_ta...', '[wpforo-breadcr...')
#3 /home/phenomlab/public_html/wp-content/themes/Impreza/framework/shortcodes/vc_column_text.php(35): do_shortcode('[wpforo-breadcr...')
#4 /home/phenomlab/public_html/wp-content/themes/Impreza/framework/functions/shortcodes.php(159): require('/home/phenomlab...')
#5 /home/phenomlab/public_html/wp-includes/shortcodes.php(319): US_Shortcodes->__call('vc_column_text', Array)
#6 [internal function]: do_shortcode_tag(Array)
#7 /home/phenomlab/public_html/wp-includes/shortcodes.php(197): preg_replace_callback('/\\[(\\[?)(wpforo...', 'do_shortcode_ta...', '[vc_raw_js]JTND...')
#8 / in /home/phenomlab/public_html/wp-content/themes/phenomlab/functions.php on line 166

//function vComp1(){
//WPF()->tpl->breadcrumb(WPF()->current_object);
//}
//add_shortcode( 'wpforo-breadcrumbs', 'vComp1' );

Dropped this function, and the errors still persist

Definitely not due to the functions. Renamed functions.php file and the error still persists.