I did a vanilla install just to make sure it's not a mod affecting this and the vanilla does the same problem:

Im having to press the attach, preview and submit button twice for them to work.

Attach and preview need a double click often but not always, but the submit button needs a second click 99.9% of the time in order for the post to submit.

The first click of the buttons seems to start the script because the page starts to display a loading icon in the tab (I.E.), but the page stalls and the loading icon shows for ages but nothing happens, until the button is pressed a second time and then everything works in an instant (like it should).