Hi Nicolò,
Please make certain that the file size is reflecting the content. As well as that you use the saved files instead of the temp back up files (the one have a tilde ~ in front of the name)
The file ~$MS_R_STOOL FOR LOW RAIL.SLDASM is only 4KB large, and it won’t open. (SLDASM)
All files that I believe should be assembled with this container are also only 4KB. (SLDPRT)
The file MS_R_STOOL FOR LOW RAIL.SLDASM is 349 KB (SLDASM)
All (sub-) files which are open with this file directly are much larger than 4KB, more in the MB range. (SLDPRT)
Please note that Cinema 4D is able to open (SLDASM) with all the parts, or just a single part (SLDPRT)
Since one of the sub-files was larger in the first group you had sent, I was under the impression that you can use it. But since you mentioned that you can’t open it while Solidworks is open, I got the impression that you use the wrong files. Please check with the Solidworks support, as all non-tilde files open nicely in Cinema 4D. At this point, I’m convinced that the Maxon Support is not the way to go.
All the best