Some users cannot upload images
 

I have over 47,000 members.
About 50 of them get an error when they try to upload an image.
They are using the same default template as the other users who upload images to the same exact board.

I never had this problem until I upgraded to this new board.
I kept an old version for years.

What could possibly be causing this error?
Could it be a browser setting on their end?

Security token is missing.

What can I suggest to them?
Thanks.
Dave.