We are experiencing similar behavior at Portal 10.4.1 with IWA through IIS and web adaptor where users encounter blank pages when opening the Portal home page or when opening web applications. In our case, sometimes the page never comes up and sometimes it will come up although it can take anywhere from 30 sec to 2 minutes. Sometimes the pages come right up fine with no performance delay. We have three Portals and have experienced it on all three, but not necessarily at the same time. It is intermittent on all three of the Portals, just as you described. If you find a solution, please post it.
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