  The gravitational microlensing technique is most sensitive to planets in a
Jupiter-like orbit and has detected more than 200 planets. However, only a few
wide-orbit ($s > 2$) microlensing planets have been discovered, where $s$ is
the planet-to-host separation normalized to the angular Einstein ring radius,
$\theta_{\rm E}$. Here we present the discovery and analysis of a strong
candidate wide-orbit microlensing planet in the event, OGLE-2017-BLG-0448. The
whole light curve exhibits long-term residuals to the static binary-lens
single-source model, so we investigate the residuals by adding the microlensing
parallax, microlensing xallarap, an additional lens, or an additional source.
For the first time, we observe a complex degeneracy between all four effects.
The wide-orbit models with $s \sim 2.5$ and a planet-to-host mass-ratio of $q
\sim 10^{-4}$ are significantly preferred, but we cannot rule out the close
models with $s \sim 0.35$ and $q \sim 10^{-3}$. A Bayesian analysis based on a
Galactic model indicates that, despite the complicated degeneracy, the
surviving wide-orbit models all contain a super-Earth-mass to Neptune-mass
planet at a projected planet-host separation of $\sim 6$ au and the surviving
close-orbit models all consist of a Jovian-mass planet at $\sim 1$ au. The host
star is probably an M or K dwarf. We discuss the implications of this
dimension-degeneracy disaster on microlensing light-curve analysis and its
potential impact on statistical studies.
