When the "new" forum came back online, I had that problem too - I couldn't do a screen shot, crop it and then directly paste it to my post like I used to be able to. I had to save the crop as a file on my computer and then upload it. However, I just tried the screenshot and paste method, and it now works. I haven't done anything at my end, so I assume the forum software now allows it again.
This was done by screenshot, selecting the part I wanted in MS Paint, copying the selection and then pasting directly into the post.