Thanks for reporting this.
I believe the problem was that your original value for logged_in_salt contained {{ which is a jinja templating delimiter. So, where the parser saw {{FT!dJpv^hlNENu it was expecting {{var_name}} but the ! is not allowed in variable names, so there was a templating error. Although the roots salts generator is awesome, other generators should work fine too, except in the rare case that the values produced include delimiters like {{.
Trellis tries to prevent interpretation of delimiters like {{ in salts and keys, e.g., as in roots/trellis#615. However, Ansible >= 2.1.1.0 introduced an internal API change (discussion at Ansible) causing get_group_vars to return nothing, causing Trellis to not take any action on the matter. In other words, I can’t reproduce your issue on Ansible 2.0.2.0, but I can reproduce on Ansible >= 2.1.1.0.