ThingWorx content loader SSL error

Hello,

I am trying to call a REST API, which is using https and certificate issued by Lets Encrypt, using Resources["ContentLoaderFunctions"].GetText

Unfortunately I am getting this error:

PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target.

If I select the option ignoreSSLErrors: 1, then I get this error:

Certificate for hostname doesn't match any of the subject alternative names

I tried to import the server's certificate to Java certstore, but it didn't help. I am running ThingWorx 9 as Docker container. What do you think can be the problem?

Thank you for your help