At any point after the file has had a material-based OTL enter the mat network, trying to create a node inside results in a hang and then immediate crash, without fail.
Things I have tried, and their results after trying to create a node inside the "Octane Material Builder":
• Creating an OTL out of the Octane Material Builder node - CRASH
• Creating an OTL out of a subnetwork of "loose" Octane nodes - CRASH
• Creating a material OTL with no Octane nodes involved - CRASH
• Adding a previously created material OTL to the /mat context - CRASH
• Adding a material OTL and then removing it from the network - CRASH
• Creating a geometry OTL - OKAY
I'm on Windows 10 using Houdini 17.0.352, using the Octane plugin build 4.00.0.15 RC5