StackState depends on a data model. When data model changes for any reason (e.g., upgrade to a new version) there is a need to run a reindexing process.
Reindex needs to be run in Terminal (or other CLI) on the machine that hosts StackState (for the production setup).
First, make sure that StackState is not running with the following command:
systemctl stop stackstate
Then, make sure that StackGraph is running with the following command:
systemctl start stackgraph
Then execute the reindex command:
sudo -u stackstate /opt/stackstate/bin/sts-standalone.sh reindex --graph default
Please note that this is a long-running process - monitoring progress in StackState logs is advised.