I opened each errored sub-assemblies separately one by one and used save. Now error disappeared. Maybe a bug ? NX 11 seems to still have some tuning.
Try turning off your partial loading in your assembly load options. For some reason this fixes our issues when we get this type of error.
Having reviewed existing cases where this has happened, there does not appear to be a single specific reason that the error is raised. I would suggest that you open an IR with GTAC and send them a copy of your NX Log File to cross reference the error against previous cases, as they might be able to isolate your problem to a known issue and supply a workaround and/or let you know if the problem has been fixed.