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When you say the tab goes blank it leads me to believe it is an HTML error and the web page is not loading properly. So when you press preview now you say it does work, but when you press post reply the screen goes blank and when you go to the thread you posted in there is no post? I am just a little confused, because you said the problem lies in when you preview the post. When you click edit you do not have to preview post.
You could also try to clear cache and cookies WeppetKo, and what OS are you using Windows 7, 8, 10, et cetera?
Go to user control panel, then go to edit option, then scroll down to Thread Display Options, do you have the third option ticked on or off, to show images? Trying reversing whatever it is set to and see if that helps.