Hi Janine,
AntConc works perfectly with LexisNexis data in German, French, Japanese, Chinese, and any other language.
It seems that you are getting all your encodings mixed up. Html, txt, xlm have *nothing* to do with encodings. They are all text-based files that can be encoded in a multiple of encodings.
The simple rule for AntConc is *set the encoding of your files in the AntConc global settings*. If you know what your files are encoded in, this is straightforward. AntConc defaults to UTF-8 (the international standard), so if you save all your files in UTF-8 from the outset, then you never have to think about encodings again.
You wrote that you saved your files in UTF-8 and then changed the settings to ISO-8859-1, which just doesn't make sense. Stay in UTF-8. (And, you cannot just "change the setting" of an HTML file, anyway. You can convert an HTML file from one encoding to another, but it is not a 'setting'.)
To help matters, I have created a program called EncodeAnt that auto-detects file encodings (so you can then set that in the AntConc global settings) and also does batch auto-conversion to UTF-8, so you can just use the converted files as is. There is a good chance that you have mixed up all your encodings and have some in ASCII, some in ISO-8859-1, some in UTF-8, and others in some other encoding. EncodeAnt can handle this and convert them all to a single UTF-8 standard.
Here is the link (note that the current version only works on Windows though):
I hope that helps.
Laurence.