I would disagree that we would recommend using IIS over our built-in web server for MDaemon. However, I have setup MDaemon to run Webmail, ActiveSync, WebDav, and Autodiscover under IIS and have not experienced the issues you've described.
I would review the client/calendar to verify if the issue is occurring everywhere or only on the particular client or calendar. I would also upgrade MDaemon to version 19.5.4 as the latest version has an updated ActiveSync engine and could possibly be a part of your issue. The release notes for this version can be found below.
https://files.mdaemon.com/mdaemon/release/relnotes_en.html If you can replicate your issue in 19.5.4, please submit a support request to have one of our technicians attempt to reproduce the issue. The link can be found in my signature below.
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Tyler Davis
MDaemon Technologies
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