This is something that just started today. I updated the site as soon as the new K2 came out and everything has been working with no issue. I created a couple of articles earlier without issue then all of a sudden with no changes my images are failing to upload.

When uploading I get the following 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." This is happening in both the backend/front end

I searched this on the forums and found the following topics.

www.joomlaworks.net/forum/k2-en/45116-solved-image-upload-error-k2-7-and-j3-5?start=40

www.joomlaworks.net/forum/k2-en/45141-solved-k2-2-7-j3-5-can-t-upload-images?start=10

Neither of those have helped in fixing the issue.

I am running the following

J 3.9.13
K2 2.10.1
PHP 7.1

Weird, because yesterday morning I could upload the images no issues. By the afternoon I went back to add a new article in K2 with the same image and it wouldn't work. I even tried from two different computers using both IE and Firefox with the same results. Even setting up a couple test article using images I've used in the past some work, some don't.

There had been no changes in the config or updates to the site during the day. I had logged out the backend after posting in the morning and when I logged back in the afternoon that's when I discovered the issue. As for the server I am pending an answer from my host at this time. I am using Apache.

My host is looking at the config this morning and has taken the site offline to do so. So if there is something you might ask on info about the site or config it may be some time before I respond back after they look into the issue also.