Encoding problems with SNAP output in QGIS/GDAL

Dear forum community
I am new to SNAP and I came across some consistently strange behaviour when using SNAP output from Sentinel 2 and 3 with QGIS 3.8.x / GDAL on Windows (Win 7 and 10). I want to export satellite imagery to a GeoTIFF and reproject / set “no data” value in QGIS in order to delete the black frame which appears after reprojecting the material to EPSG:3857. Exporting via SNAP’s GeoTIFF/(+BigTIFF) etc. drivers (32bit Float encoding) works fine, but when I want to calculate anything in QGIS, it crashes reproducibly, showing the error message “UnicodeDecodeError: ‘utf-8’ codec can’t decode byte 0x81 in position 143: invalid start byte”. The position varies, but the error message is always the same. As QGIS/GDAL works fine with other raster material, I wanted to ask whether this is a known problem and what I could try out to mitigate it. I have searched the forum first but I didn’t came across this problem. I also tried out calculating in QGIS 2.18 LTS, but to no avail - I got the same error message. In a QGIS forum entry for an older version I read that this might have something to do with the system language settings, but the bug is supposed to be fixed since QGIS 3.4.x. Being a newbie, I suppose the solution is trivial. Any help is greatly appreciated.