This requires a one-time manual step of accepting the change from testing to stable. This is basically triggered by Debian 10 becoming available.
I know one way of fixing it.
Login via SSH into your FreedomBox and run sudo apt update.
You will see a similar message as above. Not an error but a prompt with (y/n) to accept the change of the ‘Suite’ value from ‘testing’ to ‘stable’. Press y and enter.
After this all upgrades and installations should be working again.