Hey!
Those roots are in bad shape, they should be white as you mentioned there were.
When roots become brown thats means they're sick or dying..
I don't know actually what could be the cause but if you said this happened when you changed the solution.
My guess is that she didn't except the solution so well, maybe too much or too harsh or a change.
I'd try and clean those roots, change the water. Make sure the PH is correctly and go back to your old solution or try this one but less amounts.
Hope this helped.
Good luck and happy growing!