Couldn't find the answer online and spent unnecessary hours trying to solve it, so I'll plop the answer here.

Problem

Software like SDR++ can't find any devices
systemctl status sdrplay shows following errors/warnings:

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[3943]: sdrplay_apiService: UpdateDevicesL: ERROR: libusb_claim_interface() -6
[3943]: sdrplay_apiService: sdrplay_apiServiceWorkerThread: ERROR: UpdateDevices() failed 1
libusb: warning [libusb_exit] device 1.5 still referenced
libusb: warning [libusb_exit] device 1.1 still referenced
libusb: warning [libusb_exit] application left some devices open


  

Based on what I found, -6 corresponds to LIBUSB_ERROR_BUSY. The device has been claimed by something else.

Specific problem

I found this blog post on using RSP1 clone without the proprietary software: https://blog.marxy.org/2023/05/using-sdrplay-rsp1-clone-with-cubicsdr.html
It mentions blocklisting some kernel modules which capture the device.
Comparing lsmod | grep msi with known good