Freezing on Unity Editor after the secondary start play and recompilation assemble.
How can i download oldest sdk?

Unity 5.6.4.f1 and 5.6.3f1
I dont now that is wrong becouse only update sdk yesterday. And editor began to freez.
I try to change PlatformBase to old version, but it did not help me, I think the soket is not closed  and it frees a thread of the editor

maybe it's help you https://blog.tedd.no/2016/10/09/investigating-unity-hang-on-second-run-multi-threading/

i chekout commit skd V5.5.6.130 - work fine.