to use special letters in FAT32 you need to add the iocharset parameter to the usbmount configuration. This is done in file /etc/usbmount/usbmount.conf and the edited line looks like this:
Using NTFS-formatted stick it worked perfectly, maybe the mentioned parameters could be added in a future max2play-update to also be able to use FAT32-formatted-sticks from scratch? I don’t need it anymore, maybe others need it.