To be honest, I do not know.
I have attached a summary of the ClearPass' dashboard. I don't know if the number of authentication requests is particularly high. ClearPass doesn't seem to be the problem, though.
I have received some more information from my server admin. Attached is a screenshot of the processes eating up the CPU.
I think its the sheer volume of requests that is bogging down the server.
What is strange is that most of the queries seem to originate at the top of the hierarchy - which is dc=fhm,dc=de - even though dc=belgium,dc=fhm,dc=de is configured in ClearPass as the BaseDN.
I have attached a screenshot of the LDAP config as well.