I would first check that you correctly have the 'nexus-startup.cfg' in your TFTP root folder, so that it's not a path issue.  I suggest this because the uri error your getting would suggest the path is not found.  Also - you'll need to address your syntax.  Following from Nx-OS contextual help:

     tftp:  Enter URL "tftp:[//server[:port]][/path]"


Also - I would try just copying it to your flash and then to startup-config after that if it's not liking the syntax you used to copy it over.

**UPDATE:**

This known issue with your NX-OS version could be related, or even the root cause:

    CSCud05567
    Symptom : Following a system switchover, copying to bootflash on the supervisor module does not work.
    
    Conditions : This symptom might be seen under normal operating conditions for a Cisco Nexus 7000 Series device.
    
    Workaround : None.

Source is [HERE][1]

The good news is that this issue is only appearing from 6.1, so if you are in a position to move down to 6.0 family of NX-OS you should be good.  My personal preference anyway is to stay a family behind the latest and greatest unless there's a security patch.


  [1]: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/6_x/nx-os/release/notes/61_nx-os_release_note.html#pgfId-608687 "HERE"