I am trying to add a script from Juggledad in the HTML Insert:Header and get:

The following error was encountered while filling your request:
Error:504 Gateway Time-Out

This happens immediatly every time I try to do the save. Even after Re-Boot etc.

That sounds like your server is trying to pull something from another server, i.e. the one from Juggledad, and that server is not responding fast enough

I deleted your other, duplicate post