Major issue: The client in our 2 player game can't use the shortcuts. After dragging items in the slots, the UI is reacting when the corresponding key is pressed (e.g. "1"), but the character is not switching the equipped item. Some items (like bandages) seem to work (most of the time), but freshly changed items behave in the described way. Not sure if it's relevant, but the host does not have the issue at all (shortcuts work fine here).
Minor issue: Some world details differ. This starts with cosmetic things like the exploded tree laboratory: The host saw the aurora lights until the laboratory quest was completed now all there is left is some glowing sparks around the entrance. The client still sees the aurora lights during nights like the first night. Misplaced bugs (like flying water spiders or ants walking below the surface of rocks) occur on both sides and sometimes some temporary world items (like grass plates) are only visible on one side.
Except for the hardware generation, both Windows systems are quite similar (same Grounded- and Windows version, same mouse and keyboard, both nVidia GPU in different generations).