Try this CSS code: 

#wpforo h1, #wpforo h2, #wpforo h3, #wpforo h4, #wpforo h5, #wpforo h6 {
     clear:none;
}

Insert the CSS codes in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Styles Tab.  

I created an entry with the problem here: Test Entry – News, info & more – ISeeT Forum

I still couldn't figure out where it could come from :/

So for the moment I'm removing manually every header and set it as a paragraph and simply applying manually the look

When copying over some text I often have my old headers h2, h3 etc which should be conserved.

This is also no problem except if it directly starts with one - here is a big space on the top until the here in the picture h3 gets displayed.

If I put some text before as normal text this one is displayed on the top but the h3 still below my user info

I tried to figure out if there is a CSS that is creating that but couldn't find anything - maybe here some had such an issue and maybe has a solution?