After installing the plugin it worked for some time quite well, then I deinstalled UsersWP (AyeCode LtD) and after deleting the pages for profil management of UsersWP (for Login, Register, Profile, ..) I can no longer login with wpforo. I can open the community page ('forum') but can't Login, Register, and others. Instead I get a wpforo page with the warning: Nothing Found!
with the URL: https://ungeborene.de/login?redirect_to=https%3A%2F%2Fungeborene.de%2Fforum   

I get this message on debug page: 

wpForo detected wrong rewrite rules, please navigate to wpForo > Overview admin page, scroll down, click the [Soft Flush Permalinks] button, if the issue is not fixed, please click the [Hard Flush Permalinks] button.

Wrong revrite rules (current pageid is 14517):

• (?:([a-z]{2,3})/)?member(?:/|$).*$  |  index.php?lang=$matches[1]&page_id=14518
• (?:([a-z]{2,3})/)?change-password(?:/|$).*$  |  index.php?lang=$matches[1]&page_id=14518
• (?:([a-z]{2,3})/)?login(?:/|$).*$  |  index.php?lang=$matches[1]&page_id=14518
• (?:([a-z]{2,3})/)?register(?:/|$).*$  |  index.php?lang=$matches[1]&page_id=14518
• (?:([a-z]{2,3})/)?members(?:/|$).*$  |  index.php?lang=$matches[1]&page_id=14518

I performed Soft Flush and Hard Flush on Overview admin page
the file .htaccess doesn't show any (new) entry like "rewrite_rules_bk_{sec}" or a "wp_options" table,
but .htaccess-bk-{sec} files were created,
I deleted broken internal links,
I deinstalled and re-installed wpforo,
I edited the Board (there is only 1 Board with ID 0, pageid 14517), followed by hardlink flush. 
No improvement.

Just Logout by wpforo menu is possible (after login by wp-login )