try
dashboard>settings>permalinks> and just click save
@robin-w I tried it but it doesn’t work.
I had the same issue when I setup the forum page in my website Weightalogue that is hosted on WordPress.org but it wasn’t working out for me (sometimes there used to show nothing but a blank page and sometimes the page used to get a message “not-responding”) which made me de-activate the plugin.
Is there anything else I can do to make it work for my website?
Thanks!
It could be a theme or plugin issue
Plugins
Deactivate all but bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
Themes
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentyfifteen, and see if this fixes.
Then come back
As Robin suggested, you can disable all addons, i would however try disabling bbpress too.
I have had same problem lately after doing some work on my site, and i solved it by disabling and enabling bbpress.