Hello @Stijn ,
Thank you for posting here.
Here are the answers for your references.
Q1: So we tried to visit it from a Windows machine and yes also there we get the error that the certificate is not valid. Which certificate you need to bind in IIS?
A1: We can check whether this certificate is valid or not.
1.Who is this certificate issued to?
2.When will the certificate expire? When will the root certificate expire?
3.What certificate template was used to issue this certificate?
4.Is the status of the certificate normal?
5.Verify the certificate by running the command: certutil -verify "the full path of certificate file"
Q2: Second the user can only select the standard User Certificate from the web portal and not our custom User and Machine certificate. How can we change this?
A2: Check the permissions on custom certificates and ensure issued these certificate templates.
Q3: Third when the user click on the standard User Certificate, he/she can't choose the key strength and when you click submit you get this error:
Your request failed. An error occurred while the server was processing your request.
Contact your administrator for further assistance.
1.What do we mean "key strength"? Do you mean "Key Size"?
2.Can we select the CSP on the screenshot above?
3.Whether all the users encountered the same issue on all the domain joined machines?