Looking quickly through the two papers mentioned [0, 1], there is a (statistically) very solid result that something is going on with 8Be, which is only very barely a nucleus and is probably better thought of as a bound state of two alpha particles. In [0] there is a experimental observation of some kind of effect which can be explained by a 17 MeV boson. [1] looks at other experiments and finds a way around current experimental bounds by building a "protophobic" model, that is a boson that does not interact with protons. Interestingly they find that in this model they can explain the anomalous magnetic moment of the muon, if the parameters are somewhat close to already excluded limits. (Fig. 1 bottom panel in [1]) So the next generation of experiments should rule out or discover the X boson. (And actually there are many experiments which seem to have a shot at this.)
My take on this, it is probably a parametrization of something we do not understand in terms of a particle. This something may be a experimental effect or a theoretical lack of understanding or insufficient methods for calculation, or a new particle. The energy range is sort of interesting since it is close to the energy range of hadron spectroscopy, a field which is littered with 2-sigma 'tantalizing hints' of new physics. To end this, I believe it is a temptation to speculate (read I will claim I resisted the temptation until "told you so" is an appropriate response), that a heavy charged particle, cough di-photon excess, could be involved in creating the different couplings to u and d quarks.