I have just reset the enhancements on my level 19 wizard and was taking the Archmage Illusion route when I encountered the following bug for Spell Like Abilities.
Illusion II: Blur
Requires one of:
Archmage Spell Mastery I: Illusion
Archmage Secondary Spell Mastery I: Illusion
From the above description, I had the impression that either one will work so I picked Archmage Secondary first and got the enhancement Illusion II: Blur.
Whilst working my way up to Illusion IV: Phantasmal Killer, one of the requirements are Archmage Spell Mastery I: Illusion which I have not picked up yet. So I went to find it only to realise that I cannot get the enhancement anymore even though I have all the requirements met. The enhancement is greyed out and cannot be selected.
I contacted the GM and confirmed my suspicion that selecting Archmage Secondary Spell Mastery I: Illusion prevented me from getting the Archmage Spell Mastery I: Illusion. I told him that the text description of the requirements are misleading since there was no indication that selecting one will disable the other and he agreed it was strange as well. He told me to do a Feedback thread on the forum so I will do one after this.