Login 404 erorr and register erorr
Please navigate to Dashboard > Forums > Settings > Members admin page and delete all custom URLs you've set. These are wrong custom URLs set by you. All authentication fields should be clean (empty), then save it.
Then remove the empty right side are. This is wpForo sidebar and it's cached. Put some widget in wpForo Sidebar ( Dashboard > Appearance > Widgets admin page ) then delete it again. This will reset the sidebar cache and remove this empty area.
As you can read in my post it's not in Dashboard page, it's in Dashboard > Forums > Settings > Members admin page. The Dashboard is the WordPress Dashboard. Then find the Forum menu, under the Forum menu find the Settings sub-menu, click on that and find the Members Tab as it's shown on screenshot above.