Luban Slicing error when Dual Extrusion is selected

after calibration, i try to import 2 STLs to the workspace, but can’t proceed.
the 1st upload always succeeds, the 2nd always prompts this message:
“Failed to import this object. Please select a supported file format.”
this window pops no matter what the model is - if i reopen Luban and i drag the 2nd model first, it will upload correctly. it just doesn’t let me import more than one model per workspace now.

firmware versions i tried running :
4.7
4.6.3