You should begin with trying a different cables. Faulty cables are often the root of many problems.
Have you tested the Neutrik converter with another S/PDIF source. Just to make sure that it’s not causing the problem.
The next thing you could try is to connect the MR816 to anther S/PDIF equipped device. If it works then, the MR186 is ruled out as the cause, and the fault must lie with the Neutrik converter or the Genelecs.
Please, don’t be offended now, but…You are using the S/PDIF out on MR816, aren’t you? This a simple mistake that everyone, no matter how experienced, probably have made at one time or another. Especially in a low light situation.