How come "Code not validated." on the "add" page?

For a few days on all missions :
When I try to publish (after passing all tests and I have chosen a category), it states in red:

"Something went wrong. Try again to run solution in code editor before publishing it. Code not validated."

No matter what I try, it happens over and over again.
How come I can't publish anymore - or how can it be, if the code is not 'validated', that I can proceed to the "add" page at all ?