As mentioned in the main V1.47n topic, I've uploaded a V1.47n-C2 update that, while not fixing the bug, attempts to log more details when the bug does happen. (It also saves a replay, to minimize the risk of lost progress.)
If you get this bug, can you please copy and paste the contents of the generated "NeoLemmixException.txt" file here? So far, in all cases where I've been able to trigger it the suspicious part is " fRenderInterface.SelectedLemming: Exception on access attempt", but just in case anything's different in other cases, I'd like the full contents please. If you really want to, you can edit out the time it occurred.