Samet, thanks for your message.
If you call a mendix web service without authentication, it will be run under the system user session. After the call is completed, an attempt is made to remove that session, but that never succeeds and a warning is given. So nothing breaks but the warning is annoying and unnessary.
We will put a fix this on our backlog.
Just a guess here, but could it be because of how many web service users you have in the host application.
So if you called it 100K times, I'm assuming you might be running the same microflows concurrently with possibly the same user.
Would this break Mendix if you have one person logging out whilst another (same WS user) is logged in and running a microflow (webservice)?
It might help to know some of the constraints around web services from someone at Mendix, though. Also if you have more information on the number different WS Users being used, that would be helpful as well.
The last suggestion I have is to possibly consider other options like database replication instead of 100K+ calls.