Yep, my custom UI layouts are now gone.
This worked for me as well… phew, thanks for the solution
Glad to help others! 
Just had the same here too
Yup, I have learned this the hard way. I basically copy daily, my WTF to a separate saved folder out of the wow folder and delete older data in that folder slowly. ESPECIALLY around this time of release stuff.
Oh this is so annoying! Just randomly happened to me when I logged in today. Grrrr…
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Same my UI preset is gone and deleted.
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