It appears to be a bug in SR 3D Builder. The s\ has been stripped from in front of the sub-parts, which makes the file invalid. If the file loads in SR 3D Builder, then it could be argued that SR 3D Builder's file loading code also has a bug, since the parts\s\ subdirectory should not be part of the LDraw search path. (Having said that, if the program is generating files like this, then it's not necessarily a bug that should be fixed, since that would break pre-existing files even if the file saving bug is fixed.)
Also, SR 3D Builder may not support using sub-models from a sub-directory, in which case the whole thing isn't really a bug so much as a missing feature. I assume that you loaded the shortcut in SR 3D Builder directly, instead of loading an LDR file that contained the shortcut. If you add the shortcut (x581c01.dat) to an LDR file, and then load that LDR file in SR 3D Builder, I suspect that everything will be fine when you save, since only x581c01.dat would then show up in the file created by SR 3D Builder, and not the components that make it up.
If you just want to view the file on your machine in LDView, you can probably get around the problem by adding the <LDraw>/Parts/s and <LDraw>/Unofficial/Parts/s directories to LDView's search path. Do this by using the Extra Dirs menu item in the File menu.
Also, SR 3D Builder may not support using sub-models from a sub-directory, in which case the whole thing isn't really a bug so much as a missing feature. I assume that you loaded the shortcut in SR 3D Builder directly, instead of loading an LDR file that contained the shortcut. If you add the shortcut (x581c01.dat) to an LDR file, and then load that LDR file in SR 3D Builder, I suspect that everything will be fine when you save, since only x581c01.dat would then show up in the file created by SR 3D Builder, and not the components that make it up.
If you just want to view the file on your machine in LDView, you can probably get around the problem by adding the <LDraw>/Parts/s and <LDraw>/Unofficial/Parts/s directories to LDView's search path. Do this by using the Extra Dirs menu item in the File menu.