I eventually managed to get it to work. I had to uninstall TekVisa 4.1 and instead install the 220.127.116.11 version. It seems not all TekVisa releases are supported by Matlab (or vicevesa,... I am not sure). Anyway, the reason I thought of going for an earlier version (4.0.4) was that the NI-VISA and Other VISA Support from Instrument Control Toolbox support page from Mathworks (https://ch.mathworks.com/de/hardware-support/ni-visa-keysight-visa-tekvisa.html) points to it, rather than to the latest available Tekvisa version (which is 4.1).