I’m sure there is a solution for this, but I can’t find it. I can’t seem to generate preview and thumbnails for images with a resolution of 9248 x 6936 or higher. My images that are 3648 x 2736 or lower generate just fine.
Just stumbled over your topic here. I had the same for Fuji GFX 100 files or Sony A7IV, which are bigger and no previews were generated.
Edit your Nextlcoud config file and add the following lines:
‘preview_max_x’ => 3840,
‘preview_max_y’ => 2160,
‘preview_max_memory’ => 4096,
‘preview_max_filesize_image’ => 256,
The first two lines set the sizes of the preview files that are generated, I have these for a 4k screen.
The third line sets the maximum memory that is used for the preview generator. This needs more memory to generate previews of bigger files. And the last line sets a new maximum file size (in MB ig guess) for images on the input size.
At least, this works for my big images on my nextcloud.
Good luck 
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