I saw that the label you're trying to save as a new custom label is "Classification and Properties Label" (or the equivalent in GER version). This label works by pulling out properties of the attached element (in this case, the wall). If you save this label again as a custom label (regardless of whether it comes with another 2D element or not), the function of the label will be lost, therefore it cannot display anything.
When I tested with Archicad 23, the same thing happens, so I don't know whether it works in any previous versions or not. Could you confirm whether the same workflow creates a usable label in previous versions?
Unfortunately, I was still unable to create a working label using the given file on AC23 GER and INT, running different builds (5010 and 6004). I will ask my colleagues for more info about this. As soon as I have an answer I will let you know!
I asked the help of my colleagues to see why the label creation worked for you but not for me. It turned out that this function worked on Archicad 23 GER, but it never worked on INT version. In Archicad 24 GER, this function also did not work. Let me ask the developers to see why it behaved like this. As soon as I have an answer I will let you know!
I have received an answer from the developer: saving an associative symbol label as a GDL object is not prohibited by AC, but it is strongly contraindicated because the result is unstable. Most important technical reason: labels receive some of the important information through GDL globals, which have wrong values in the saved Library Parts when placed as labels (their "original" values were hardcoded into the saved Library Part's GDL script). In the Classifications and Properties label's current implementation, this might not cause a problem, but other labels (or this label's later implementations) might show absolutely bad information after saved as a new label libpart.
The workaround here is: with associative labels, if you uncheck "Hide with associated element" in the Lable's Settings Dialog. So if you uncheck this, the new label will appear with the correct saved label.
I hope this explains the situation. Please let me know if you have any further questions!