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Thanks. I cleaned up the function by declaring the return value properly as Double (previously it may return Double or String), but that alone (?) did also not yet work.
I found another conditional Formatting in my sheet which also referred to a custom function and after changing it to a conditional formatting which depends on a cell, the Bug seems to be gone.
In Conclusion: Excel2013 does not support own VBA-Functions in conditional formatting anymore :(
Work around: Use own function to calculate relevant things in cells and refer with the conditional formatting to these cells. Not quite sure whether that always solves the problem, but it solved my problem.