Hi, Nick. welcome to DC.
What happened is that the emulsion broke. You end up with solids and liquids separated.
If you still have the mixture, remove it from the FP bowl and add raw egg to the FP and pulse it to break it up. Then slowly add back the old mayo a little at a time and it will re-emulsify.
Five minutes is a very long time to add oil to a mayo. A slow trickle is important at the start but as you proceed, you can pour a little faster then faster still. It shouldn't take more than a minute for 250 ml. The oil should go into the FP in a steady stream while the FP is running.
Try adding a teaspoon or two of mustard along with the eggs. That will aid in the emulsification process.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan