Problem with certificate versions?

[ghost]

TL:DR:
I have two different certificates when I look into them with
openssl x509 -in cert.pem -text
one shows me "version: 3 (0x2)" which has the subjectAltNames set and one shows me "version: 1 (0x0)" in the top line. The one with version 3 is not working, but this is the one I need. I have my own CA and this I imported to mqtt-explorer but "validate certificate" works on neither of them (both are signed with the same CA).

Long version
I am currently at the installation of my production environment. For this I installed mosquitto on my server and tried around in the conf in order to have both secured mqtt and websocket listeners. For this I started to generate the needed certificates (CA and for the mosquitto itself). So far so good I achived with a first setup to make it work with mqtt-explorer as the only client.
Then I tried to make one of my two client implementations to work (nodejs with mqtt.js). This gave me several errors because of the certificate. Even though I installed on my manjaro the CA like it is recommended in the docs, I had to set NODE_EXTRA_CA_CERTS and point to the CA certificate. So far so good. In the end I only stuck at the problem, that the "subjectAltName" was not set in the first mosquitto certificate (version 1).
After a lot more tries I finally achieved to create a other certificate with the correct altNames set (the version 3 thingy). But now mqtt-explorer does not work with it. I switched between the to certificates and it is cleary that it only works with the "version 1" cert.
In the end I let the mqtt listener have the "v 3" cert set and the websocket listener the "v 1" and connect to the websocket with mqtt-explorer. The behavior when I try to connect to the v3 cert is that the icon with "abort" is flickering in about 1 s freqzency and it alternates between "connection with healt offline" and "connection with health connecting". In the bottom left corner it shows "Disconnected from server"

[ghost]

I got it working when I set the mosquitto to tls version 2. But I dont really get why.