Maybe investigate BUG-000106057 - I have had just had Esri "support" tell me this is my issue with our cache on S3 not working using export map or WMS calls (although it does work using rest tile calls through the JS API). It seems this doc saying it was addresses at 10.5.1 was premature. https://support.esri.com/en/download/7514 We tried switching to a key which our info sec guys aren't very happy with but that also failed. Frustrating as now we are so far down the migration we will be swapping out to EBS storage at 10x the price - and we are talking 10-20TB.
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