@Robert Keogh Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
It could be intermittently issue, I was able to upload the file in Archive Tier through Portal and Azure Storage explorer
If the issue still persist, Please provide the below mentioned information.
The platform/OS version you're running on? Is this happening to only particular file? What is the file size?
Un-install and re-install the latest Azure Storage explorer click here
Please let us know if you have any further queries. I’m happy to assist you further.
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