We are now using SanDisk ULTRA cards and they are working fine as well.

Dear Alex (and to whom else it may concern): apperently, my problem was related to the sd cards. I bought new ones (same specification as the old ones but a different brand) and audiomoth is now working just fine. Alex, thank you very much for your promptness and attention. Best regards, JV

@Alex Rogers If the SD is the root of the problem, red light should be on when the device is recording?

@Alex Rogers see a picture in attachments.

@Alex Rogers Batteries (Duracell Alkaline 1.5V and MicrosSD HC 32GB).

Actually I have deleted it already. But I will send you a new one in a few hours.

Version number of the firmware are 1.2.0 in both advices.

The version number of the configuration app is 1.2.2. Sorry for my ignorance, but how can I access information on the device firmware?