There is a login issue with the X-Plane Client. If you wish to comment please do so in the thread in the X-Plane forum

https://www.fseconomy.net/forum/x-plane/129421-can-t-connect-dom-parser-error

How to Fix

Please update your Python version to 2.7.17- download from here: https://www.python.org/downloads/release/python-2717/
Please make sure to pick the amd64 version and install for All Users if you're running X-Plane 11 or X-Plane 10 64 bit (amd64 is just an acronym for x86_64). X-Plane 9 and X-Plane 10 32 bit users please chose the x86 version.
The installer will handle the update correctly if your previous installation of Python 2.7.9 was done correctly as well.
NB: Do not try to use Python 2.7.18; it broke the client in previous tests.