Have you seen any errors when using Constant Contact? If not, is this something you are running into only when working in a newsletter?
At times, if we are only seeing this error when working in a newsletter we can try creating a fresh campaign instead of copying the same template or removing blocks and adding new ones in.
Are you having issues with any other websites? If so, would you mind running a speed test at Fast.com to ensure this is not a connection issue?
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