Stu, I have now resolved this but would like you to review what I did to ensure it's correct.
I found the problem was to do with your code in the preset file, notably two extra \ and a false statement which I changed to true. I've pasted your code below and highlighted what I deleted/changed in red.
s = {
id = "0021553D-A04D-4F24-9281-948DB70C33FC",
internalName = "Mirror L-R",
title = "Mirror L-R",
type = "Export",
value = {
collisionHandling = "ask",
["com.adobe.lightroom.export.imagemagick_comman d"] = "convert {INPUT} -flop \\( +clone -flop -crop 50%x0+0+0 \\) -geometry +0+0 -composite {OUTPUT}",
["com.adobe.lightroom.export.imagemagick_group"] = 0,
["com.adobe.lightroom.export.imagemagick_reimpo rt"] = 1,
["com.adobe.lightroom.export.imagemagick_stack"] = 1,
exportServiceProvider = "com.adobe.lightroom.export.imagemagick",
exportServiceProviderTitle = "ImageMagick",
export_bitDepth = 16,
export_colorSpace = "AdobeRGB",
export_destinationPathSuffix = "imtesting",
export_destinationType = "sourceFolder",
export_useSubfolder = false,
extensionCase = "lowercase",
format = "TIFF",
initialSequenceNumber = 1,
jpeg_limitSize = 100,
jpeg_useLimitSize = false,
metadata_keywordOptions = "flat",
minimizeEmbeddedMetadata = false,
outputSharpeningLevel = 2,
outputSharpeningMedia = "screen",
outputSharpeningOn = false,
reimportExportedPhoto = false,
reimport_stackWithOriginal = false,
reimport_stackWithOriginal_position = "below",
renamingTokensOn = true,
size_doConstrain = false,
size_resolution = 240,
size_resolutionUnits = "inch",
tiff_compressionMethod = "compressionMethod_None",
tokenCustomString = "",
tokens = "{{image_name}}_mirror",
tokensArchivedToString2 = "{{image_name}}_mirror",
useWatermark = false,
watermarking_id = "<simpleCopyrightWatermark>",
},
version = 0,
}
Thus, the new code looks like:
s = {
id = "0021553D-A04D-4F24-9281-948DB70C33FC",
internalName = "Mirror L-R",
title = "Mirror L-R",
type = "Export",
value = {
collisionHandling = "ask",
["com.adobe.lightroom.export.imagemagick_comman d"] = "convert {INPUT} -flop \( +clone -flop -crop 50%x0+0+0 \) -geometry +0+0 -composite {OUTPUT}",
["com.adobe.lightroom.export.imagemagick_group"] = 0,
["com.adobe.lightroom.export.imagemagick_reimpo rt"] = 1,
["com.adobe.lightroom.export.imagemagick_stack"] = 1,
exportServiceProvider = "com.adobe.lightroom.export.imagemagick",
exportServiceProviderTitle = "ImageMagick",
export_bitDepth = 16,
export_colorSpace = "AdobeRGB",
export_destinationPathSuffix = "imtesting",
export_destinationType = "sourceFolder",
export_useSubfolder = false,
extensionCase = "lowercase",
format = "TIFF",
initialSequenceNumber = 1,
jpeg_limitSize = 100,
jpeg_useLimitSize = false,
metadata_keywordOptions = "flat",
minimizeEmbeddedMetadata = false,
outputSharpeningLevel = 2,
outputSharpeningMedia = "screen",
outputSharpeningOn = false,
reimportExportedPhoto = false,
reimport_stackWithOriginal = true,
reimport_stackWithOriginal_position = "below",
renamingTokensOn = true,
size_doConstrain = false,
size_resolution = 240,
size_resolutionUnits = "inch",
tiff_compressionMethod = "compressionMethod_None",
tokenCustomString = "",
tokens = "{{image_name}}_mirror",
tokensArchivedToString2 = "{{image_name}}_mirror",
useWatermark = false,
watermarking_id = "<simpleCopyrightWatermark>",
},
version = 0,
}
The same changes in the Mirror R-L file work too. Hope this helps...