Your tree looks to be in much better condition than I was fearing from your initial description.
Serrisas can be fussy trees. They don't like changes. Change in light, change in temp, change in roots, and they can drop all their leaves. The key is consistency. Additionally they have weak roots that are prone to root rot if you don't have them potted in well-draining soil and a well-draining pot. They like a lot of water, but do not like wet feet.
Your leaves are really dark green (which is good). The browning on the tips could be due to (1) a sudden shift to much brighter light (2) fungus (though it doesn't look like it to me) (3) leaf burn due to salts in the soil / too much fertilizer / bad water.
I see a lot of dark green leggy growth. Is the tree being heavily fertilized?