My Cinema 4d is pretty customized in terms of plugins and scripts - I'll have to make another install which will be faster than removing everything, I guess. That is something I can't do today, since there's still some animation work to be done, but I will do it tomorrow morning.
Concerning the problem being related to Octane there might only be a possibility - I imagine - concerning the implementation of information stored in Octane-Objects of any sort. For instance, here is a file http://www.philmaron.de/privat/clothfrost_003_test-r18.c4d which has only 5mb when saved in R17 (old Octane), also 5mb in R18 with Octane removed from Plugins folder, but 560mb when saved with newest Octane Plugin in R18. This means there is a lot of information strored in the scene as soon as Octane is enabled - which might be causing some sort of hickup. Not sure, but it is a possibility, or am I wrong?
My Version is R18.048 (RB197636).