Plus is slow at rendering. Doing the same file local(15s) vs using crowd render (37s). This step of updating objects flags takes a long time. The scene is simple, some sphere with subdiv and some donuts and 2 lights.

I can make it work with 2.9.0 but there are some issues with the sync process for example if i click render during sync, nothing is going on. If i close off blender during sync : "Blender quit" message in a cmd window plus other errors. A nice feature would be to pack the scene in order to not get pink renders in case textures where involved.

Try a ping and see if the it works. If is not working, disable from the windows defender window , Firewall & network protection the private network. You might, as me, downloaded the free version of CR that only works with blender 2.9 and you want to make it work on blender 2.9.x. I am waiting for a reply to see if this is the issue before going for the payed version.