Quote from TristanianIt's not, it just happened that you had that addon embed Ace2. As for the actual error, I'm not at all sure if the SecureHook syntax used is valid (I'm also not entirely sold on the fact that a secure hook is needed here to parse a simple message). It is more logical to assume the obvious, the UIErrorsFrame_OnEvent function was changed from 2.4 to 3.x to pass a 'self' argument before the event and the message. Player Notify is simply not calling the original with the proper arguments, so nothing is displayed.
Indeed, noticed that too when I was testing what I made for a bit longer. But also searching a bit more, I found out that you had already posted a working solution for the problem.
So, to correct out the first one with the wrong fix, I'll attach the correct one here, which is your fix from the other thread.
Credits for the following file to Tristanian, it's his fix, not mine,