Is this happening with or every file you try to open?
Error 18003 (similar to error 84003) often occurs when trying to union objects that are currently grouped as Select the objects and type to return them to their base 3D solid state. 3. Adjust Geometry (The "Nudge" Technique) autocad error 18003
: If the error persists during a UNION command, try joining smaller groups of solids rather than everything at once. This helps isolate exactly which piece of geometry is causing the "migraine". Is this happening with or every file you try to open