According to this Post article accompanying the poll, "Support has rebounded since July among moderate and conservative Democrats, while Republican opposition has also softened."
The improvement may only seem like a few percentage points. However, the fact that two other recent polls -- NBC/Wall Street Journal and Democracy Corps -- have also shown an uptick in support for the ACA suggests the change is likely authentic (for now, at least).
The rise in support seems surprising in the context of difficulties with the federal website for people to sign up. However, some of the state websites (where such are available) seem to be working well.
The larger driver of public opinion toward Obamacare may be the unpopularity of the Republicans' actions during the recent federal-government shutdown. Paradoxically, in seeking to register their staunch opposition to the ACA, the GOP may actually have made it more popular! It is perhaps for this reason that former Florida Gov. Jeb Bush, a Republican, was calling on his party to exercise more "self-restraint."