Have you tried to contact the official RICOH customer support? They likely have a bigger database with support queries. I think you have a good set of information to report. It seems like your configuration and versions are up to date.
This THETA Stitcher is new and a bit tricky to use.
Have you tried using the trial version of PTGui as a test? I realize that PTGui is a bit pricey, but it may help you to experiment with the DNG images while you sort out the stitching. Information on PTGui is here.
This is likely unrelated, but the Windows machine I used for testing does have an external NVIDIA graphics card and I’m using the NVIDIA drivers. The machine used in the test only has 16GB of memory, same as your machine.