I was having big weird issues with Toast Titanium 16 on MacOS X 10.13 High Sierra. The toaster icon didn't bounce during launch; the icon stayed in the dock but with no dot under it; no Toast processes showed in Activity Monitor; it took an unbelievably long time to even settle into this or any state; sometimes the whole GUI froze. Corel support gave me this fix within 24 hours, which worked perfectly for me (iMac 21.5" late 2012):
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