One possible cause for this behavior is that the water column has dropped before the pump. It is quite possible that the error does not occur if the pump is switched on again, but only if it has been in standby for a longer period of time.
To rectify the fault, we suggest the following procedure:
On the suction side, it is best to install a non-return valve at the end of the suction hose/suction pipe or, if it is not possible for structural reasons, as close to the water as possible. If a non-return valve is already in use on the suction hose, check whether the water column can be held by the pump over a longer period of time. If the water column is not maintained, clean the non-return valve or replace it.
If it is not possible to use a non-return valve, the suction line must be 100% vacuum-tight.
To check whether the suction line is vacuum-tight, we suggest the test described below:
Start the pump and hold the end of a pressure hose installed on the outlet side in a bucket of water. If you keep seeing air bubbles, the suction side is clearly leaking. The suction line must be sealed. The video on the GARDENA YouTube channel, which you can find at the following link, can also help you:
https://www.youtube.com/watch?v=REtXXM7gcnw