I tested it on a AIQUM 9.10. Not sure what the issue is but my code still works against the latest version: https://github.com/matthew-beattie/AIQUM . Try the GetUMClusters.ps1 in the repo and see if that works. I noticed the .NET Certificate code i used is different from your code. Hope this example helps?
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What client OS are you running? I recently encountered a similar issue for a customer running a legacy OS (Windows Server 2012 R2) resulting in the error "Error The request was aborted: Could not create SSL/TLS secure channel."
I took a packet trace using wireshark and compared the client OS supported ciphers used to negotiate with AIQUM and found that the ciphers configured in AIQUM did not support the client OS (hence the authentication failure). I enabled TLS 1.2 on the client and configured the AIQUM ciphers to append "TLS_RSA_WITH_AES_256_GCM_SHA384" to the "tls.ciphersuites.selected.v12" configuration and rebooted both the client OS and AIQUM. I was then able to successfully authenticate to the REST API from the client OS.
The following KB articles might be helpful to enable the diag user in AIQUM so you can SSH to it and update the cipher configuration.