undecodableError: invalid start byte when processing SRTM data

Downloading SRTM-Data using the QGIS-SRTM-Downloader plugin works fine. I now do want to process this image data further by using the contours plugin within the extraction tools of raster data, but there’s always an error message:

undecodableError: ‘utf-8’ codec can’t decode byte 0x81 in position
154: invalid start byte

How to solve this problem?

Browsing the web told me that there was once an error with utf-8 related to imagery date, but this should have been solved with release 3.4 (I’m using 3.16).

The same error message also appears when trying to clip the SRTM-image into with a polygon to reduce its extent to a smaller area of interest.