Corrupted user profile DACLs (Discretionary Access Control Lists) can block AutoCAD.
Try moving one of the objects a tiny distance (e.g., 0.0001 units). This often allows the Boolean operation to complete by resolving mathematical ambiguity. 4. Use the BREP Command To remove the history of a solid and simplify its geometry: autocad error 18003
command to revert them to individual 3D solids before trying the operation again The "Micro-Move" Fix autocad error 18003
Repair installation