Geany documents (in memory, not files on disk) are UTF-8 so on opening a file has to be converted to UTF-8 if it isn't already. The list above is the process Geany uses to try to do this. Its not simple, encodings __cannot__ be uniquely determined from file contents, so the process above is attempted.
Encodings are evil hangovers from a pre-Unicode past and should be burned with fire. There is no way to tell what encoding has been used on a file, if I write a file with my locale encoding it may fail to open with your locale if its different, or it might open with different characters. _Use Unicode!!!!_ [end rant at the world in general]
The only message I get in geany is; syslog does not look like a text file or the file encoding is not supported.
As it says, all encodings known to Geany failed to convert to utf-8 without errors, see item 5. on my list.
Encoding conversion libraries are system provided libraries, not part of Geany, and they do not provide brilliant error indications, neither where nor what went wrong. So there is no information about what the encoding conversion doesn't like about your file.
BOM
https://en.wikipedia.org/wiki/Byte_order_mark