This story ended in a deal with the local ElectroVoice rep.
We had to pay for a new tweeter, as the coil had turned blue due to ??? and that ??? caused an open circuit. The amp was OK.
We also found out that the unit was sent to Germany for repair because it was cheaper to have it done down there.

We still have no idea what has caused this. We have not experienced any pops, scrunch or screeching feedback - but we will for sure live with the rule “amps on last” and “off first” because it’s the only thing we aren’t sure we have followed. Maybe a mic or line cable has been connected or disconnected to the mixer while the monitors were turned on?

Well, it's the end of this story, the thread is now closed.

Best regards
Aulis G