The log tab of the Contour tool gives a hint about what happens
GDAL command output:
0...10...20...30...40...50ERROR 6: Range of levels [90,inf] not supported
Gdalinfo shows that your test image has nodata set to positive infinitive
Band 1 Block=2000x1 Type=Float32, ColorInterp=Gray
Min=29.008 Max=70.435
Minimum=29.008, Maximum=70.435, Mean=35.163, StdDev=4.438
NoData Value=3.4028234663852886e+38
I am not sure about what values there really are in the test raster. If I unset the nodata value by making a copy with gdal_translate -a_nodata none
the statistics show odd results
Band 1 Block=2000x1 Type=Float32, ColorInterp=Gray
Min=29.008 Max=inf
Minimum=29.008, Maximum=inf, Mean=-nan(ind), StdDev=-nan(ind)
Anyway because you did not set the nodata value through the user interface the gdal_contour starts to create a contour for interval [90,inf] and fails. I do not know why it fails because "inf" should be understood by GDAL as we will see later. But it seems also to be impossible to set the nodata value into either "inf" or to "3.4028234663852886e+38" through the user interface because QGIS is changing the input in both cases automatically into number "999999999.0".
What does work is to capture the gdal_contour command, edit it is run it from the command line.
gdal_contour -b 1 -a ELEV -i 1.0 -snodata inf -f "ESRI Shapefile" test_LPr1.tif demtest.shp
Result looks good
For GDAL 2.4.0dev "inf" is the right way to give the source nodata. I had a try also with the nodata value reported by gdalinfo 3.4028234663852886e+38
but GDAL did not understand that it means the same as inf and it started to create contours for each meter which made the memory usage to skyrocket and finally Windows crashed with blue screen.
As a conclusion you made a mistake when you did not give the nodata value and QGIS has a bug because it is impossible to give "inf" as a nodata value. There may also be an issue in gdal_contour because it does not consider numeric 3.4028234663852886e+38
as infinite float32 value.
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