When I reply to a post, I get an error page HTTP Error 500
Then it says I'm posting too fast if I hit the back button on browser.
First of all please click on "Mark all read" button located in forum footer.
Also, I'd recommend you read this support topic:
You have some PHP error in website. Please disable plugins and check it again. Also, please navigate to Dashboard >Forums > Tools > Debug > Errors & Issues Tab. wpForo displays all errors from other plugins and from your theme. The errors shows files where they come from so you can see the problem maker plugin or theme file.