In addition, on my site, visitors can add topic as guests but on add-topic page they can't see the topic form without logging in. 

Please navigate to the Dashboard > Forums > Usergroups > edit the "Guest" usergroup and make sure the "Can access to add topic page" CAN is checked.

I was lazy about trying to deactivate plugins one by one. It's the "WP-Optimize – Clean, Compress, Cache" plugin. It worked after I deactivate it but then I just activate it and wpforo shortcode is still working. Weird to me. I still cannot edit the page although tried deactivating all plugins. I guess it's also about my theme files or something.

In addition, on my site, visitors can add topic as guests but on add-topic page they can't see the topic form without logging in. 

There is no such issue on our test website.

It seems there is a plugin conflict. So, please try to deactivate the plugins one by one and check after each deactivation to find the problem maker one.

I'm trying to solve what's wrong with my site but I'm out of ideas. I make new page, write the title as "add-topic" and copy paste the shortcode as HTML. So there's no even <p> tags. 

Then, all I got is a blank page with only the title. I have tried it with forum id too, still the same result. Interestingly, I also cannot edit the page after I publish it. Again a completely blank page. I had to delete and remake the page on every trial.

By the way, I have a contact page and it's working with its own shortcode. No problem for it.

I changed my theme. Deactivate siteorigin page builder. Clear the cache everytime. Translate the add-topic title and tried it like that. In the end, nothing helps.

I see the add-topic code and the page is working here. So what's wrong with my site? Any idea?