Interesting. This may be due to one of many attachments inside of the R15 parts of the rig that are not used.
This only happened to me with the right leg R15 parts, but after deleting the problematic attachments inside of them it worked fine. Try deleting the problematic attachments inside the R15 parts. Here’s where they’re located:
If you delete all of them, I can guarantee you will run into problems. Just delete the ones for the body parts that are disconnecting.