This picture is normally shown on heat</ lightsteress,,, but 21 degree wont be to warm
look for your feedingsolution, my be a sign for rootproblem too ( look for th ´e temperature of the solution)
and if you think you overfert. delute your feedingsolution.
You have to check every factor of your grow to find out what is happening.
Atm iam growing one plant that shows a similar picture, and its either to warm or to bright, she just tends to folding the edges.
But believe in your intuition and iam sur you find the reason
happy finding