error trying to connect: invalid certificate: UnknownIssuer

[u63725]

I use the Anki Linux client with the version 2.1.48 and Anki Syncserver with the latest Docker version. I have create the ankisyncd folder in ~/.var/app/net.ankiweb.Anki/data/Anki2/addons21/ankisyncd with __init__.py file. I am using HAproxy with Lets Encrypt as reverseproxy.
The content of init.py looks like this

`import os

addr = "http://anki.example.com/" # put your server address here
os.environ["SYNC_ENDPOINT"] = addr + "sync/"
os.environ["SYNC_ENDPOINT_MEDIA"] = addr + "msync/"
`

When I start Anki I get the following error message: ⁨error sending request for url (): error trying to connect: invalid certificate: UnknownIssuer⁩