Atlassian Connect - Set entity properties in Jira/Cloud

I develop an AC/SpringBoot Jira plugin which needs to store properties in the host application. Atlassian provides Hosted Data Storage which I like to use.

I ran a test via Firefox RESTClient to play around with this feature.

When issuing a GET request to this URL:

https://kuespert-dev.atlassian.net/rest/atlassian-connect/1/addons/<MY-PLUGIN-KEY>

while being logged in to my Jira/Cloud instance on another tab everything is fine: I get back information about my plugin, including the key.

However, when issuing a PUT request to set a variable (like described in the documentation mentioned above) using this URL:

https://kuespert-dev.atlassian.net/rest/atlassian-connect/1/addons/<MY-PLUGIN-KEY>/properties/testProperty

and this request body:

{"string":"string-value","number":5}

as well as the required header:

Content-Type: application/json

I get back this response:

{
    "status-code": 404,
    "message": "Add-on with key does not exist."
}

This is a bit irritating, since the first REST call did find my plugin while the second to the sub-resource did not.

A similar question was already asked on the 'Atlassian Community' site, but no solution was found until today.

Maybe it's just a wrong error message or something similar. Does anybody here have experience with the 'Hosted Data Storage' feature and can help?

Thanks, Matthias