The problem is most likely not specific to bbPress and is with your server settings or htacess. WordPress may not be able to generate or modify the .htaccess file for your website due to file permissions issues. It might help to delete the htaccess file, grant correct permissions and refresh permalinks which will generate a new htaccess file. Try to enable php error log in htaccess as well.
# enable PHP error logging
php_flag log_errors on
php_value error_log /path/to/htdocs/PHP_errors.log
Create a blank PHP_errors.log file in the directory specified in your .htaccess file and set its permissions to 777. Now refresh the page that’s giving the 500 error and check if that log file now contains an error(s).