ubuntu – What dictates whether I should use sec=ntlm or sec=ntlmv2 in my mount options?

I’m mounting the same cifs share on two Ubuntu machines using the same credentials. On one machine, I have to use sec=ntlm in the options, or it fails with mount error(22): Invalid argument; on the other I have to use sec=ntlmv2, or it fails with the same error.

I initially thought that it was the version of the OS that dictated which I would end up having to use, but I now realise that both machines are the same OS: Ubuntu 16.04.7.

If it’s not the OS version, what is it that means I have to use sec=ntlm on one and sec=ntlmv2 on the other? It seems random but I’m sure it’s not!