Type "/console taintLog 1", wait for the error, then look at "Logs\taint.log". This file is plain-text and is written in real-time, there is no need to reload the UI to see changes. New stuff will be at the bottom (end) of the file. If nothing is there, type "/console taintLog 2".
FYI: "1" will log all illegal taint (calling secure functions, improperly hooking secure functions, and/or actions that may lead to altering the functionality or value of a secure variable) while 2 will log every little thing that happens regarding taint (every variable declared by an addon is insecure and, therefore, tainted by design - this will be in the log file).
I got the following information:
An action was blocked because of taint from MACRO_TAINT - CastGlyph()
10/27 16:30:51.975 Interface\AddOns\Blizzard_GlyphUI\Blizzard_GlyphUI.lua:585 GlyphFrameSpell_OnClick()
10/27 16:30:51.975 GlyphFrameScrollFrameButton2:OnClick()
I´m not really sure what to make of it though.
Edit: It seems like the same problem occurs with the addon "Broker: Micro Menu" enabled. So I guess the problem with oPie is my micromenu ring. I just wish I new exactly what the problem was.