We present ground-based transmission spectroscopy of the highly irradiated and ultra-short period hot Jupiter WASP-103b using FORS2 on the VLT. The light curves show significant time-correlated noise which we model using a Gaussian process. Our observations reveal a featureless spectrum across the full range of the FORS2 observations which is inconsistent with previous signs of Na absorption and strong Rayleigh scattering. Using the additional data from Gemini/GMOS, WFC3 and Spitzer observations we perform a Bayesian atmospheric retrieval on the full optical-infrared transmission spectrum and recover a detection of H2O absorption in the near-IR. We conclude that our FORS2 transmission spectroscopy observations are most easily explained by a high-altitude cloud deck or hazes masking the absorption features in the atmosphere of WASP-103b. Our results demonstrate the continuing potential of ground-based observations for investigating exoplanet atmospheres and emphasise the need for the application of consistent and robust statistical techniques to low-resolution spectra in the presence of systematic noise.