Fuck, I forgot. Again! Thanks for bugging me. I have no documentation, I just worked on it till my stick was working. I'll have to try to get a new one running again, noting the steps I did. I'll try to find some time at the end of next week. Honestly I was SURE that the developers would switch the build tool to a newer syslinux version immediately, which should have fixed that problem. They are the ones getting paid for it, not me...
If the developers would want to do a fix for this, they really only would have to use a newer Syslinux Version in their USB Creator. This should be straightforward to implement. The Problem with my method is that I create an official USB stick and then try do replace all the files of syslinux that I can find with the newer version afterwards, trying to skip the files with unraid specific configuration. I thought it'd be easy to recreate with this Information. But if someone overwrites to much it'll not work if someone overwrites to less it'll not work. Problem is I don't have a clue which files I replaced. The developers shouldn't have this problem. As replacing the files didn't work for "Yourself" maybe it's only the bootsector. I'll have to do some tests.
The developers don't seem to care. Maybe the percentage of old Dell's is to small. I'm just using my stick and I'm at the moment not risking doing regular updates. (Because of this it's time I do a manual update anyway) I hope I don't fry my Array by doing it.
As I don't have Arch on my main machine anymore I'll try to script this in WSL as platform independent as possible which will be better for others to reproduce.
I also have no Idea which version of syslinux it was, As I was using Arch back then it could have been a version compiled from AUR. I remember that i found an information which syslinux version fixed it, i'm sure it was an rather old one. The syslinux files in the repository of the USB Creater were updated 2 Years ago, so no one tried that. As the download on the website still points to the old 1.6 release it wouldn't help much anyway. I could try to update the USB creator myself, but, I'm not setting up a whole developement environment to fix this!
I'll however try to create a patch script which will get the stick created with the USB Creater tool, to boot on my Dell. I'll have to, to get my stick to the new version anyway, as I simply don't know if an update over the webinterface will fry the fix, or not.