Enable connector debug and IDM debug, perform the operation and look at the IDP server log and IDM server log. You might get an idea why this error is thrown by IDP.
We are using this function and we have a Change handler which fetches manager details from LDAP using a Plugin.
Another option is to create a task in IDM for user selector and map the task to the user selector in IDP (I don't know how to do this )