Sorry, I thought you saw something that literally said fatal error.
If there are no messages and ACT simply closes, check the Windows Event Viewer for application log crashes. There will probably be two per crash and only one of them has useful information... so grab both.
I can only guess that the FFXIV Plugin, OverlayPlugin or Cactbot are reading process memory incorrectly. There's not really anything else I can guess at here because it is not widespread.
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Any thoughts?