Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 660 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 445.87
Broken: version: 2.90.0 Beta, branch: master, commit date: 2020-08-09 12:07, hash: rB00162e6b7b0f
Caused by rBeee35ebdfbed: Make `BKE_scene_copy` fully handle all duplicate modes, deep copy included.
rBb05fa123d741: Make `BKE_collection_duplicate able to handle master collections. might also be relevant here
Seems like blender hangs in BKE_id_new_name_validate / check_for_dupid
Exact steps for others to reproduce the error
Move default cube to Scene collection
Try to make full copy of scene.
It happens with custom settings and factory settings too.