Azure backup stopped working (InvalidQueryParameterValue: blockid, not base64 string)

[jaredthirsk]

My daily backups stopped working around May 19 (might have been a few days slow on the update.)

ERROR: Microsoft Azure API: Fail: Code: 400 Value: Value for one of the query parameters specified in the request URI is invalid. details (if any): <?xml version="1.0" encoding="utf-8"?><Error><Code>InvalidQueryParameterValue</Code><Message>Value for one of the query parameters specified in the request URI is invalid. RequestId:423e14c8-701e-00my-guid-abcdef000000 Time:2018-05-22T17:44:09.4920166Z</Message><QueryParameterName>blockid</QueryParameterName><QueryParameterValue>1844a0bfe3bmyguid12345605222718b000001</QueryParameterValue><Reason>Not a valid base64 string.</Reason></Error>.

I am using a classic storage account (because last I checked, the RM storage account wasn't supported by backwpup.)

Someone reported this issue on the wordpress.org support forum: https://wordpress.org/support/topic/azure-backup-fails/