If the preview client of InterFormNG2 is unable to connect with the InterFormNG2 host, then you can use the section: Preview client problem: Login failed.
A part of debugging connection problems is also to look into both the system log, but also the log of the preview client.
This log file can be found in a path, that depends on data folder setting during install of the client, but the default path is:
C:\ProgramData\InterFormNG2\profiles\default\logs
Inside of this directory you can find the preview client log file called: remote_preview_client.log
The level of details found in this log file depends on the log level, that you have specified in the client