Are you in charge of the server or is someone else? Is it possible that some setting has changed since you last connected? I assume you are using the latest version 8 packages on both Windows and Linux sides. The error looks like that the server has been configured to not allow users who are not NX users.
Can you check how the following keys are set in server.cfg? Is it set to 0 or 1? 0 is the default setting when you install.
# Enable or disable NX users DB:
# 1: Enabled. Only users listed in NX users DB can login to the NX
# 0: Disabled. All the authenticated users can login.
# If the NX user DB is disabled, any user providing a valid password
# from local DB or through SSHD authentication, can connect to the NX
# system. This is likely to be the default when SSHD authentication
# with PAM is enabled.