update: i had some time and did a fresh installation of Xubuntu 18.04.3 on an USB SSD. Suspend works with Kernel 5.0, first i suspected the nvidia driver but it still works after installation of the nvidia drivers. So for me it is either a setting which i'm not aware of that is different or the main difference between these 2 installation: my Laptop runs zfs root, the USB installation is simple ext4.
update: i had some time and did a fresh installation of Xubuntu 18.04.3 on an USB SSD. Suspend works with Kernel 5.0, first i suspected the nvidia driver but it still works after installation of the nvidia drivers. So for me it is either a setting which i'm not aware of that is different or the main difference between these 2 installation: my Laptop runs zfs root, the USB installation is simple ext4.