@meteop Hello, API key does not change no matter how many stations you have. Stations are recognised by their station ID, your first station will have ID 0. You need to add this parameter into Weewx as well.
These "errors" are due to Personal Weather Stations. You can verify by clicking them. They are labelled pws or madis. They are not official weather stations. Windy cannot delete them as they have been added by their owner.
So it would be worth having the possibility to select in Settings only "official" weather stations (labelled wmo, airport, ad, ship...)
Even if the Windy API key were smaller you still would not be able to upload to Windy with your Ecowitt console using the "Customized" server upload feature because it uploads using a different protocol than what Windy uses.
My suggestion for you is to use the Ecowitt GW1000 or Froggit DP1500 in conjunction with your station if you don't currently have one of these. It will be compatible with your current station as long as you get it with the same frequency as your current station (433, 868, or 915 MHz).
Then you can run one of the follow software applications that are capable of getting data from the GW1000 / DP1500 and uploading it to Windy. These compatible applications are: Cumulus MX, Meteobridge, Weather-Display, and WeeWX. A good solution to run these is a Raspberry Pi. If you need more guidance you should head over the wxforum.net and read up on these solutions or ask for help there.
This is not really surprising. This weather station is a Personal Weather Station (pws). Some of them give bad figures but Windy cannot delete them.
Weather stations labelled madis and pws are non official weather stations.