In the Horizon dashboard, I tried to access the instances section, which is part of the compute service. I got this error :

JSONDecodeError at /project/instances/
Expecting value: line 1 column 1 (char 0)
Request Method: 	GET
Request URL: 	http://localhost/dashboard/project/instances/
Django Version: 	3.2.10
Exception Type: 	JSONDecodeError
Exception Value: 	
Expecting value: line 1 column 1 (char 0)
Exception Location: 	/usr/lib/python3.8/json/decoder.py, line 355, in raw_decode

I checked all the running devstack services, they were all running but one :

I tried to start this service only but it was not enough. I had to restart all services :

sudo systemctl restart devstack@*