The Wi-Fi Plant Sensor blinks red when it was unable to transmit data to the Koubachi servers. Please check your Wi-Fi network by visiting a website, for example www.koubachi.com. If this fails, you have a general connectivity issue that must be fixed.
If your internet connection is OK, you may press the button once to force the sensor to transmit data. Often this resolves the issue. If not, check your router settings that the supported Wi-Fi standard still includes 802.11b or 802.11g (Fritz!Box routers were reported to automatically update their firmware and change that setting).
If the Wi-Fi Plant Sensor keeps blinking red after the above checks, please re-configure the sensor. To do so, visit http://go.koubachi.com and follow the well-illustrated instructions in the configuration assistant.