That actually did the trick, now it says:
solace.messaging.errors.pubsubplus_client_error.PubSubPlusCoreClientError: SESSION CREATION UNSUCCESSFUL. Untrusted certificate. {'caller_description': 'do_connect', 'return_code': 'Not ready', 'sub_code': 'SOLCLIENT_SUBCODE_UNTRUSTED_CERTIFICATE', 'error_info_sub_code': 99, 'error_info_contents': "Session '(c0,s1)_default': The peer certificate is not trusted, rc='unable to get local issuer certificate'"}
There is a working Let’s Encrypt certificate active inside the broker.
I link to the default Ubuntu CA Trust Store
When I use without_certificate_validation() the connection works fine! So thank you for that at least 