Mobile app error: 403 on PUT for json

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Hello, I have tried to link the mobile app to my passbolt account, and the application prompts with “Something went wrong”. Looking into the logs I got this (censored for security reasons):

Device: Google Pixel 5
Android 12 (31)
Passbolt 1.3.0-4

9:35:09 AM --> PUT http/1.1 (41-byte body)
9:35:09 AM <-- 403 Forbidden (37ms, unknown-length body)
9:35:09 AM retrofit2.HttpException: HTTP 403 Forbidden
    at retrofit2.KotlinExtensions$await$2$2.on

Seems like the problem is a 403 Forbidden. I am using a self-hosted installation, with Let’s Encrypt certs (I have seen the help post about self-signed certs, but I don’t think that applies to me).
Additionally, in passbolt.php I enabled the mobile feature like so (did that long time ago):

    'passbolt' => [
        'plugins' => [
            'mobile' => [
                'enabled' => true
            'jwtAuthentication' => [
                'enabled' => true

Any help would be appreciated.

Possibly related: What ports does the mobile app use during setup?

I think you may need to set the User-Agent header in your JSON request. Try adding a line in JSONFunction.makeHttpRequest() before the call to execute() like:

httpPost.setHeader(“User-Agent”,“Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/43.0.2357.81 Safari/537.36”);

or, on the line before calling setEntity() you could do something like:

para.add(new NameValuePair(“User-Agent”,“Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/43.0.2357.81 Safari/537.36”));