We solved this problem by switching to the ArcGIS Web Adaptor (Java Platform) 10.8.1, running on the Apache Tomcat web server. It does not experience this error at all. It's possible there's a bug in ArcGIS Web Adaptor (IIS) 10.8.1 that's causing this. I don't think the error could be caused by the IIS web server itself because we've used the same IIS web server with previous versions of ArcGIS Web Adaptor (i.e., 10.6, 10.7, and 10.8), without the error occurring.
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