I suspect it's because the image is in CMYK/8 format. Maybe Opus only understands RGB/8 PSD images, but can still read the embedded thumbnail, which is probably always saved in RGB format.
It would appear to be a CMYK-format image which is not currently supported by Opus for PSD files. We'll try to get support for this into the next version.
Also while probably not a factor in this problem you might make sure you have the option "Maximize Compatibility" turned on in your Photoshop preferences.
This is really an issue now. I'm being sent images that are sent saved either in CS4 or with the Maximize Compatibility turned off. I cannot open these files in the DOpus viewer.
Is the PSD viewer being updated to include the new photoshop format?
Support for CMYK Photoshop images -- what the root post is about -- was added a long time ago.
You may have been sent images that don't have a flattened version within them. Virtually nothing, except Photoshop itself, can view those files. To view them you have to know how to interpret, render and combine all of the layers and effects in the PSD file which is a huge undertaking.
If people turn off the "maximize compatibility" option then the flattened version is not saved into the file; only the layers are stored. It's not specific to CS4; it's been that way for a long time (although the name of the option has changed over the years).
You'll see this if the file doesn't have a flattened version. The message doesn't come from Opus; it's saved into the image by Photoshop. (The message is the flattened version of the image, so that programs which display the flattened image display the message saying the real flattened image is not there.)
If you instead see a message saying the file "is invalid or not a recognised picture" then please provide a sample image to reproduce the problem.