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:

[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 system (Arch Linux), sure enough, these aren’t loaded while in Linux Mint they are:

msi001                 20480  1
msi2500                36864  0
videobuf2_vmalloc      20480  1 msi2500
videobuf2_v4l2         36864  1 msi2500
videodev              352256  3 videobuf2_v4l2,msi001,msi2500
videobuf2_common       81920  4 videobuf2_vmalloc,videobuf2_v4l2,msi2500,videobuf2_memops

Solution

Add the following lines to /etc/modprobe.d/blacklist.conf to blacklist these kernel modules:

blacklist msi001
blacklist msi2500

And reboot.

Personal note/rant

I spent so much time… I tried different API versions, so I also had to uninstall the newer ones which with this stupid software can only be done with careful manual removal of files.
I tried different USB ports (just in case), reinstalling SDR++, SoapySDR, installing some other potentially interesting looking libusb related packages, trying to find at least one other person with this issue…

And in the end it’s just interfering kernel modules like with RTL-SDR.
Well, if this gets indexed the solution should be easy to find based on the error now.