in reply to
Couldn't start TLS: SSL connect attempt failed with unknown error error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
I did a search on "14090086" at my personal-favorite search engine, DuckDuckGo, and found a number of hits that I did not examine too closely. But one thread here at LinuxQuestions.org did seem to get fairly close to the point and to a course of action for diagnosing it.
First of all, there is a more detailed error-message, which is like yours:
Peer certificate cannot be authenticated with known CA certificates: SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
Then, it was suggested to “Try to get verbose information from openssl s_client -host host -port port.” A number of other related suggestions followed, along with the expected small amount of snarking about this-or-that, but it is clear that this thread was on to the essential problem right away. In the final post of that thread (#15), their user says that he solved his problem, and gives a fair number of details as to how he did it. HTH.