The galaxy SDSSJ0952+2143 showed remarkable emission-line properties first reported in 2008 (paper I), which are the consequence of a powerful high-energy flare. Here we report follow-up observations of SDSSJ0952+2143, and discuss outburst scenarios in terms of stellar tidal disruption by a SMBH, peculiar variability of an AGN, and a supernova explosion. The optical spectrum of SDSSJ0952+2143 exhibits several peculiarities: an exceptional ratio of [FeVII] transitions over [OIII], a dramatic decrease by a factor of 10 of the highest-ionization lines, a very unusual and variable Balmer line profile including a triple-peaked narrow component with two unresolved horns, and a large Balmer decrement. The MIR emission measured with the Spitzer IRS in the narrow 10-20mu band is extraordinarily luminous (3.5 x 10^{43} ergs). The IRS spectrum shows a bump around ~11mu and an increase towards longer wavelengths, reminiscent of silicate emission. The strong MIR excess over the NIR implies the dominance of relatively cold dust. The X-ray luminosity of 10^{41} ergs measured with Chandra is below that typically observed in AGN. Similarities of SDSSJ0952+2143 with some extreme supernovae suggest the explosion of a supernova of Type IIn. However, an extreme accretion event in a low-luminosity AGN or inactive galaxy, especially stellar tidal disruption, remain possibilities, which could potentially produce a very similar emission-line response. If indeed a supernova, SDSSJ0952+2143 is one of the most distant X-ray and MIR detected SNe known so far, the most MIR luminous, and one of the most X-ray luminous. It is also by far the most luminous (>10^{40} ergs) in high-ionization coronal lines, exceeding previous SNe by at least a factor of 100 [abridged].
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