Do you have copy of wpForo template files in your WordPress theme? Please check in WP active theme folder. If you have /wpforo/ folder with wpForo template file, backup those and delete this dir.
His Robert just checked and there are wpforo template files in the WP theme folder.
Ok, all is clear. You use old wpForo template files. It seems you've done some customizations. They will be lost. You should delete this folder to allow wpForo use new template files from wpForo plugin directory.
Also you can copy the new files (only the files you're customizing) from wpForo plugin folder (wpf-themes/classic/) , customize those again and overwrite in WP theme's /wpforo/ directory.
I can't understand why people don't read the Update Notes before updating wpForo. All is already mentioned in the release summery:
Sorry Robert I was rushing the post above as I had to nip out and made a mistake when typing.
To be clear, I did not find any template files in the WP theme folder whatsoever and I have not done any customisations. So I still need help figuring this out.