After talking with the admin the problem was solved by replacing the ASUS WL-330N3G with a TP-LINK TL-MR3020 and an USB hub.
At the moment it looks like it´s an issue of the firmware of the ASUS WL-330N3G - it´s not talking with the weather station.
So I only can recommend not to use the ASUS WL-330N3G for the PCE-FWS 20 or any other File Offset WH-1080 clone until this issue is fixed.
Data request problems with PCE-FWS 20 (WH-1080)
Moderator: Mattk