An update on this - first, the error message provided from Exchange is super generic, which stinks. That said, we were able to discover and reproduce a non-environmental issue that we can fix on our side related to how passwords are communicated from AC to Exchange (part of the reason why we suggested a simple password as a workaround, and why that has been working for others). This is likely the cause of at least some of the reports here on Thwack, and potentially yours, for this error that so far haven't been attributed to environmental or config changes.
We're preparing this fix to release with our upcoming release, but that release isn't scheduled to hit the street for another ~6 weeks. What we're going to do in the meantime is package up the solution and you can install it as an upgrade to your instance of AC and validate whether this is in fact the cause of your issue. If by chance it's not, we'll keep troubleshooting. It will take us a little while to finish our validation of the fix since it widely affects all EWS connections, and as soon as we've got it we'll let you know. Feel free to ping back here if you don't hear anything by the end of next week and I'll update status.