First, thanks a lot for the help so far. I'm still trying to fix this issue, and it's really nice to be able to talk to some experts.
Maybe set "min_tls_version = 1.2"
I tried adding "tls_min_version = 1.2" (as I suppose that's the correct spelling of that option), but it didn't change the behavior. It still works on versions before #8e54822d..., and fails on versions after and including #8e54822d... However, I found out that adding auto_chain = no does indeed change something. It doesn't cause the newer (failing) versions to work (which is what I tested yesterday), but it also causes the older (working) versions to fail as well. Maybe our certificate chain isn't in order? What order is needed?
In the end, this is a certificate / OpenSSL issue. FreeRADIUS just calls OpenSSL for the SSL magic. If there's an issue with that, it is very rarely the fault of FreeRADIUS.
But if it's an OpenSSL-issue, how can changing the freeradius build version have an impact? Doesn't it need to be somewhat related to freeradius, if downgrading fixes the issue? And I thought that the error message "unknown CA" is issued by the client, as the client is rejecting the certificate when using newer versions of freeradius. Am I wrong? Thanks again for your time, Lukas