The large photo (you posted) shows it as "uncalibrated"
Are you exporting to JPG using sRGB?
Check both the export dialogs (Presets?) are they both using sRGB.
Thanks for the very quick and quite likely answer to the problem.
I have never changed this.
Where do I check and if necessary, change the setting?
This doesn't happen if I export to the same folder and don't change the dimensions.
I think I've found the culprit and it's pretty dangerous if you don't know about it, which I didn't.
Export with previous !
It not only remembers folder location,sharpening and dimensions but it remembers presets too.
So one day you export a photo with a preset and sometime later you use "export with previous" it will apply the same preset.
I think that's the way it works and comments would be more than welcome.
The "way it works" I think is also rather cryptic, and 'help' is rather minimal-
Quote from the Lr Help Reference file-
Export photos using the previous settings
You can export photos using the same settings from the most recent export session that was set manually, including modified presets. Otherwise, the Export With Previous command does not work with export presets.
"Export with previous" works exactly they way I expect it to.
It is exactly like going to the Export dialog, changing nothing, and pressing the ok/go/export button.
Since I don't usually remember what the previous settings were, I never use "export with previous".