You can try and troubleshoot using the following steps:
Check if curl can connect to your Cumulocity tenant (curl uses the OS Certificate truststore by default, so it usually an easy way to test if it is trusted or not)
For example assuming your tenant is called thin-edge-io.eu-latest.cumulocity.com, you can do a curl against the tenant, below uses the tenant/loginOptions as this returns some json instead of html on the console (which more readable)
If curl fails, then you will need to follow general instructions on how to add a certificate to the truststore, e.g. entrusted-certificates-installation
If curl works then it most likely that you are using an older version of thin-edge which does not use the OS truststore by default. If you can upgrade thin-edge, then upgrading to 0.11.0 will fix the issue.
You can verify which thin-edge version you have using:
tedge --version
If you can’t upgrade thin-edge for whatever reason, then you can should be able to set the Cumulocity root certificate setting.
Assuming your OS stores the certs under /etc/ssl/certs/ca-certificates.crt, then update the config settings to:
sudo tedge config set c8y.root.cert.path /etc/ssl/certs/ca-certificates.crt
# Try connecting again
sudo tedge connect c8y
Though generally upgrading would be the better option as you will also get all of the nice features that come along with every new release
curl https://"custom-domain"/tenant/loginOptions
works fine
tedge version 0.11.0 is installed
The problem seems to be related to our custom domain. I tried a test instance we still use, that one works fine. Are there any known issues with that? Probably our certificate?
Ah, yes the custom Cumulocity IoT domain feature will be causing the error. The custom domain is only specific to the http communication (not mqtt).
Support for using separate http and mqtt endpoints for Cumulocity was added in 0.11.0. You can read a bit of background on the topic in the original issue #1931.
But you can set the Cumulocity IoT http and mqtt endpoints as follows:
sudo tedge config set c8y.http "mycustom.domain.com:443"
sudo tedge config set c8y.mqtt "mytenant.cumulocity.c8y.io:8883"