Thank you for your response. The FB number is FB15619515. I appreciate your assistance in resolving this issue.

In addition to the investigation of this issue, we found the following from our IIS logs: When the communication timeout occurs, a "timer_sslrenegotiation" error log is output. By enabling "Negotiation Client Certificate" in the server's SSL settings, the error log is no longer output, and the timeout does not occur. We suspect that iOS 18 does not support renegotiation or that some other cause is preventing renegotiation, resulting in the timeout.