[Acrobat-Preflight fails for ICC profile check]

6 hours ago, thomaso said:

Yes. This is as it may and should be.

Whereas in the screenshot above you see the same .afpub as PDF/X-1a. With 1 profile, 1 color space only. – So, obviously it works both.

It may have one profile -  the CMYK output Intent - but it also has multiple color spaces, so they were not converted to CMYK.

[Acrobat-Preflight fails for ICC profile check]

It appears that you have a custom Output intent "ISO Caoted v2 (ECI) "Custom)", and yet multiple pictures (bilder) in CMYK, RGB, and Black (schwarz). IF all images were converted to CMYK you would not see RGB and Black pages in the page list, nor Gray in the color list.

[Acrobat-Preflight fails for ICC profile check]

PDF/X is based on an error in thinking by the engineers. This is obvious by the logic behind the implementation. IN PDF/X assigning a CMYK profile to the document removes profiles embedded in any images. This untagging of individual images can lead to color errors if the PDF/X document Output Intent is different from the original color space of the document images.

A universal CMYK document output intent is also problematic because every offset press has its own color space - ISO standards are just "standards" we use to measure from when profiling a press. When a press is "profiled" they will have there own unique color space based on the inks used and the substrate printed on, combined with the characterization of each press.  PDF/X sets one standard based on a Color Profile that will have nothing in common with any real world press.

This discrepancy is the same one that affects the Pantone Matching System. To faithfully reproduce a Pantone Solid Color all the print specifics listed in the Pantone Swatch Book will need to be met exactly - paper type, ink type, and press. This never happens for Pantone or for "standard" CMYK profiles. 

Because of these limitations, it is best to color manage each image individually, embed a color profile into each image and export with no conversion and preserve the image profiles, and not use PDF/X.  The pre-press department at the printshop will then be able to confidently color manage the file for their press.  When we receive PDF/X the first thing we do is remove the Output intent because all it does is add a layer to proper color management and can be a false flag.

[Color Management]

This may not seem like a big deal to most people, but as a color specialist who manages color from submission to press, having grayscale support in a publishing program is awesome. Adobe only supports grayscale in Photoshop, which can lead to many headaches in InDesign when grayscale is converted automatically to Device N black. Device N black can lead to color management problems in pre-press due to not being an editable, raster color space. So kudos to Affinity for properly handling the grayscale color space - that one thing tells me you are far beyond Adobe in the color management arena.