Florian: Looking at the file Bypass.png, it shows no connection to VDDB. Per Table 11 you need VDDB to be fully powered or the internal dc to dc converter will be off and Viso will go to 0V. Please check that the VDDB pin has at least 12V during the conditions you are having the problem. Also, check the waveform of voltage for the other supplies to the ADuM6132 VDDL and VDD.
Regards, Brian