[Alex]

The thing is, the floppy image is created from scratch with some special cooked-up programs, that understand FAT and simply add the two files kernel.map and readme.txt (sysload.sys is already there). Since everything is done in this program, I'll have to have a look into that special program (that writes the FAT) and see why it isn't keeping both FATs in sync. Since for the GRUB version (which hopefully will be out before May) uses a modified version of the same tool, I'm not sure if it makes much sense if I try to modify the outdated version of it. on the other hand, I'll have to see if the same error isn't ported over to the new version.

For the ISO file the solution has been found: The Joliet entries were one byte too short, or put in other words, stored in UTF16-Big Endian. This may have been a misunderstanding on my part when I added the joliet support, but there's no real good documentation around for it, so it was kinda messy. In the hex print, it looks like this:
[pre]
14 00 6b 00 65 00 72 00 6e 00 65 00 6c 00 2e 00 6d 00 61 00 70
[/pre]
Which means: the name takes up 14 (hex) characters, and the characters are 006b, 0065, 0072 etc (kernel.map). At least that's what I tought it meant.

Apparently, it means something different, namely the lenght of the name is 0014 (14 00 in little endian) characters, and all other entries are, also in little endian: 6b 00 => 006b, 65 00 => 0065, ... etc.
At the end you probably guess correctly, 70 xx which should be converted to xx70.
What I now do: add an additional 00 at the end and at least my 7z accepts the iso files from now on.