I have not managed to find a decent explanation for either of the error codes you received. All I can suggest is that you test with a Unicode and ASCII version of the string, and be sure that the path to the file is correct.
I'm trying to solve one problem with custom cred.prov. on win8.
The problem is that if you select any tile (e.g. my, custom) and you have CPUS_UNLOCK_WORKSTATION scenario - Logonui will dismiss it automatically after 1 minute. This is not relevant to CPUS_LOGON scenario. Selected tile will stay untill user press Ok or Cancel buttons.
So I'd like to ask, maybe somebody has worked with such thing already, is it possible to handle that moment when selected tile is going to be dismissed.
using debugging I found that IConnectableCredentialProviderCredential::SetDeselected() is not called when IConnectableCredentialProviderCredential::SetSelected() is called by selecting the tile.