The Antec VP450 does NOT have active PFC: http://www.hardwaresecrets.com/article/Antec-VP450-Power-Supply-Review/1487
although it is otherwise a very good PSU.
Looking at the PSU page, red for 10 seconds looks like an overload.
Most likely, you need a larger UPS. They are more expensive, but if you can afford it, I'd recommend a true-sine wave unit.