Win 10 Multi user using client

I have 2 users in Win10 setup using the Sync Client (V3.1.1)
The Client has it´s own user to log into the server. (used for both win user)

I have user a (parent) who has lots of folders to sync. Here everything works fine.
I have user b (child) where only the folder for the child should be synced. This does not work and I get the error “write protected folder” - which is not true.

I could not find any doc which explains how to set up correctly the client for multi user usage.

Maybe someone has done it and could show me how to setup.

I want the windows user to do:
user a: sync all folders
user b: sync only one folder

if this is not possible - I can use different user for a server login - but how to setup that in the windows user?