Iray Server 2.5 does not support the Bridge Protocols necessary to work with NX12 and 1847.
Solution
Currently the latest version of Iray Server that can be used with NX is V2.43.
NX uses the protocol version "Iray 2016.3 - Bridge Protocol 278300.3661" which is not currently supported by Iray Server V2.5.
The next functional release (FR) of NX available mid 2019 will use "2018.0.1 - Bridge Protocol 302800.1185" which is supported by Iray Server V2.5.
Customers should avoid using the automatic update option within Iray Server as this will apply a version later than V2.43. If this has occurred then a downgrade will need to be performed.