No, I didn't remove those folders or edit the reg keys. Sigh... I guess I should do that....   Thank you for your help, I wouldn't have thought about it. I tend to assume that when the fix for something is so often 'reinstall the program' that the uninstall routine actually removes everything that would interfere with that, but I know that's just a dream...

Oh, and I don't know how to edit the reg keys.

When you said "completely uninstalled", did you mean you have deleted Reallusion folders (I would delete it from Programs and Content location as well as clear Temp folder) after uninstall. Because most uninstalls leave traces behind.
Also registry. Not sure if you're proficient with registry editing. But I would clean Reallusion key from both HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE (that is if no other Reallusion products are installed on the computer)

Have completely uninstalled CC3 and components and reinstalled twice now. Still getting the exact same problem. Any suggestions would be welcome.

Well, after getting not much in the way of feedback from RL, I reinstalled CC3. Hub was a little wonky, but smooth compared to the first time. However, the problem remains the same. Any ideas? I've sent a screen cap to support. I'm really losing my mind on this.

Game Base has 73 bones as oppose to Normal CC Base, which has 101 bones, including those double *skin* bones. It is just easier to set an IK chain for Game Base, if you need it to pose bones.
Rigify gives you an automated IK chain I think (I did not play with it much), as I do not have any experience posing/animating in Blender though. I animate in iClone.
But on the other hand you can rig an OBJ online in Mixamo. Just less headache.

I see- I thought it was about poly reduction so I didn't bother investigating the game base- especially since it has problems. What are the advantages of this CC rig? Do you have any advice about posing it in Blender? If it's a lot of work I'll just have to wait until the game base exporter is fixed or I can solve the OBJ problem and just rig with Auto Rig Pro. It would definitely be convenient to have a useful rig transfer automatically from CC3 though.

Yes, that is normal, that is why people want to use a Game Base character armature. It is free from *skin* bones.

No problem. Just one more thing. I remember someone had some weird problem after overriding a standard default base.
Worth to check: https://forum.reallusion.com/FindPost451527.aspx

And thanks, I appreciate your help.

Btw, the bones in the FBX I've exported from CC3 to Blender don't look anything like the bones in the original CC3 model- for instance, there are two bones in the upper arm and two bones in the forearm. Is this 'expected' behavior?

I submitted a ticket, but my screenshot wouldn't attach... what else?