In recent work with high-resolution grating spectrometers (RGS) aboard XMM-Newton (Pinto et al. 2016), we have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that, in some directions, such winds can become optically thick enough to block and reprocess the disk X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01 - 0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disk. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The spectral variability of the source shows a softening of the spectrum at lower (~ Eddington) luminosities which agrees with the scenario of wind clumps crossing the line of sight and partly obscuring the innermost region where most hard X-rays originate. The tentative detection of a soft lag is consistent with a picture where the outer photosphere of the disk is reprocessing higher energy photons emitted in the innermost regions. ; Comment: 17 pages, 16 figures, 6 tables, submitted to MNRAS, comments welcome