Oh they already tried to fix this issue in a previous update? okay. Well, I'm very glad to know that they are already aware of this issue.
I did try all the options at the Academy's gate. They require at least 5 Persuasion to pass and I have 0. My reputation with him was actually very low (8 Attitude), though I feel like it should be high. My reputation was very high with my other 2 companions.
As you said, I probably missed other things too. Missing only that one encounter with Aetera shouldn't be enough to lose him. But that one was probably very important to him.
Fane's Attitude towards my main character was only 8 at the gates of the Academy. But I have no idea what else could I have missed, at least the mask quest I did.
I actually stumbled upon the back entrance before trying the front gate of the Academy. But I wasn't ready to enter it yet, so when the characters started their dialogue about entering and their portraits at the top left of the screen disappeared, I just quickloaded and went to do something else. Later on, I entered by the front gate.
Today I loaded an old save file to see if I would still lose him entering by the back entrance and yeah, he still leaves. The dialogue is the same as going by the front gate. Maybe they changed this on an update? idk.
But don't worry about it Raze, I think this is really a lost case. If the developers are aware of this issue, our job here is done.
Thank you for at least trying to help. You're always very helpful.