It seems that UBB code doesn’t handle QUOTE tags within other QUOTE tags well.
There’s another way to nest comments, but it involves CSS. People comfortable with coding might use this approach or a variation on it.
Put the material you want to quote in a DIV. Then nest the DIVs as needed.
For example,
first div blah blah blah.
second div blah blah blah.
third div blah blah blah.
should produce this:
first div blah blah blah.
second div blah blah blah.
third div blah blah blah.
But don’t forget to close those DIV tags!
HTH.