Hi, after inserting image from server using media manager I get known error:
The image was not uploaded. Please ensure that you used a valid image file and that file uploads are properly configured on your server.
Upload function works fine the problem is with adding image to an article.

It works fine with v2.7.1.

Any idea?

It's Joomla 3.7.5 on PHP 5.6

Thx! Bostjan

Just tested with J2.7.5 K2 2.8 and php 7
Upload from setver and local machine to item image both worked without error.
Also works when file is uploaded via media manager
Could be your image file or php version