We were also unable to use Open C$ and Open Admin$ custom actions
For us it looks as though it is a problem with windows explorer.
We are using Explorer++ instead
copy the executable to your lansweeper share
these custom actions should work
{actionpath}Explorer++.exe \\{smartname}\Admin$
{actionpath}Explorer++.exe \\{smartname}\c$
Works well for our environment
If logged in as an elevated user Open C$ and Open Admin$ custom actions are no problem here
We are logged in as a restricted user
We are using alternate elevated credentials to launch the browser
We have two domains with an elevated user for each domain
Hope this helps someone
EDIT: It turns out that our problem was associated with group policy not properly applying to the clients. But Explorer++ worked without the policy on the client.