weird signs on serial monitor

I’m playing around with first examples in arduino ide and rephone.
The upload of a sketch works without any error, but the serial monitor shows only weird signs in endless order:
On line contains:

• starts with: ÿ
• then a sign from the ascii table
• then somthing I cannot paste here

This goes through the hole ascii table and start from begin again.
But my “Hallo world” does not appear

Any hints?

Maybe this issue is caused by my forgotten firmware update on the rephone

But - the firmware-update does also not work - as described here in the forum by other guys.
No Im looking for an old win32 installation …

You’ve likely had the wrong baud rate selected at the bottom right of the serial monitor. Whatever it says in your code sample (ie 115200) select that rate in the drop-down.