sounds strange!

can you tell how full your +drive is approximately? Maybe this is what happens if it’s filled up.

Re loading to project: what happens if you create a new project with empty sample list - can you load samples into this project?
Could be that sample RAM is full despite not using all slots, this would explain why you can’t load samples to the project.

But the +drive thing is weird, unfortunately the Rytm does not report what’s going on in detail…

Have you contacted support?

About file format: this doesn’t matter, PCM data is simply packed into chunks defined by the SDS protocol, and then translated back to raw PCM by the receiving side…