Templating Variable not working in Panel

I have a little bit “strange” problem here. A “HOST” variable is defined and it works as expected in the panels, execpt one.
i removed and readded the panel several times, same with the variable config but this does not help.
the panel works when not using the host variable but setting host manually.
grafana is version 6.5, influxdb some 1.7 version.
any hint whats going on here?

variable config:

not working panel query: