Solved: Forgot to go through Open EUICC. 
Hi!
I am still on DivestOS (planning to move, currently nothing sensitive goes onto the FP). Changed carrier, and deleted the old one’s eSIM. Now the “+” sign is inactive and I can’t add a new eSIM.
The old one has worked flawlessly. There is also a (working) physical SIM installed (may be irrelevant, because removing it does not make a difference).
Is there a fix for that?