Hi Marpet, thanks for helping!
I opened SNAP up again today and the same problem appears for both L2A and L1C data - however, Sentinel 3 imagery does open when downloaded from the same source. I’ve attached an image showing the LST loaded in whilst a Band from the L1C image brings up the same error as before:
Data loaded in:
L2A downloaded from CREODIAS, as the original is in the Long Term Archive of Copernicus Hub:
S2B_MSIL2A_20200112T111329_N0213_R137_T29SPC_20200112T122412L1C downloaded from Copernicus Hub:
S2A_MSIL1C_20210613T112111_N0300_R037_T30VVH_20210613T133154Sentinel 3 Level 2 LST downloaded from CREODIAS:
S3A_SL_2_LST____20200120T100602_20200120T114701_20200121T182831_6059_054_065______LN2_O_NT_004.SEN3
And the .Jp2’s seem to load normally into QGIS without issue (Band 8A from the same L2A S2B folder as listed above):
SNAP also reports it is fully up to date, as are all the plugins. I’m afraid I can’t give much more information to narrow down the issue, it was a literal night-and-day change when it stopped loading in the Sentinel 2 data.