Hi,
I just wanted to provide an update that the issue was indeed with SELinux. It had been disabled a long time ago, and sestatus reported that it was disabled (as I showed in a previous comment). However, it seems that, somehow, it became active again, even though it was still reported as disabled. Several other issues appeared alongside ThinLinc, all pointing toward SELinux. Re-disabling it fixed everything, including ThinLinc.
Thanks for your assistance!