Thanks again for your quick reply! And also thanks for Mnemosyne in general. It is a great tool and I use it every day!
At my phone, there was a folder /Android/data/org.mnemosyne which contained only a subfolder 'files' which in term was empty. I deleted that folder and, while my phone was connected to my Linux computer, made a search for "Mne" in nautilus to make sure that I didn't miss a folder but did not find any other folder.
Do you know any other location where data from Mnemosyne could be stored?