I have the same problem.
But it seems to be limited to when you reply to an existing post.
When you start a new question or post you can upload a file, but when you reply to an existing post to can not upload an image.

I find that rather limiting and makes replying sometimes almost impossible.
Hopefully this can be changed quickly

Hi Bart, 

For Jesús the issue was solved over time, without any actions taken from my end.
Could you please tell me in what forum and possibly what post you can't insert images in your replies?

That was yesterday when I was working on a reply in the XPP forum:
community.sdl.com/.../is-this-expected-behaviour

Thanks for letting know.
And what about today? Is it working for you?

I just tried again in the same discussion thread and today (as it is a Thursday?) it does work and it gives me the correct window with the upload button and the file/url entry:

While yesterday (because it was a Wednesday?) it did give a window with only the label 'URL' and no clickable upload text below the entry box. (yes, I should have taken a screenshot...too late now)

Any idea why this could have been the case? 

I just tried again in the same discussion thread and today (as it is a Thursday?) it does work and it gives me the correct window with the upload button and the file/url entry:

While yesterday (because it was a Wednesday?) it did give a window with only the label 'URL' and no clickable upload text below the entry box. (yes, I should have taken a screenshot...too late now)

Any idea why this could have been the case? 

Don't worry, I believe you even without a screenshot 
Thanks for explaining.

Bart Terryn said:

Any idea why this could have been the case? 

I don't have the answer to that question yet, investigating as we speak.
Hoping to have an answer to this shortly!