I'm inclined to guess this is user/config error but I'm stumped and need a hint.
A few weeks ago I had things working with Library Server running at home and connecting thereto from work. With authentication enabled. I'm on port 52199, have tried several builds, am now on 73 (on both ends). With authentication disabled everything works. If I enable authentication at home and restart MC at work and try to connect I get the password dialog, after typing the password I get "failed and will load default instead". Have tried resetting password several times in case that was it. Including setting it to nothing at all and to a single letter. Failure dialog is very fast (almost suspiciously) to come up. Then turn off authentication, restart MC again, all fine.
On a complete from-left-field guess I tried forwarding UDP as well as TCP traffic on the same port. No help.
I briefly thought this was related to a build update. But I've tried going back to 58 (earliest I had lying around) (on both ends) and problem still repro'd.
If I try connecting thusly from another machine in my house (no internet, no router, no port forwarding) it works (with auth enabled).
Sigh. Feeling dumb.