[dt_imageio_large_thumbnail] error: The thumbnail image is not in JPEG format, and DT was built without neither GraphicsMagick or ImageMagick. Please rebuild DT with GraphicsMagick or ImageMagick support enabled.
[dt_imageio_large_thumbnail] error: Not a supported thumbnail image format or broken thumbnail: image/tiff
Subsequent invocations of dt with and without the -d imageio option produces no error.
Looking at the image files which I had asked dt to import (via ‘add to library’), none of them are of a file type that the manual says requires GraphicsMagick support, so I assume the error is caused by a defect in the image file. How do I figure out which image caused this error report from dt?
Without more information, any guess about what caused this is just that: a guess.
That said, the messages you posted indicate that the thumbnail itself was the cause: a wrong format, or (imo more likely) somehow corrupted. Or it may have been an error in reading the thumbnail file.
That subsequent runs of dt gave no error can have several reasons:
if you do not use a disk cache for thumbnails (not the default), the offending thumbnail didn’t exist anymore…
it’s possible that those runs didn’t need the offending thumbnail file (thumbnails are generated in the size required if absent from the cache).
you edited the file concerned, causing a new thumbnail to be generated.
You can always delete the images from the disk cache while dt is not running.