I’m guessing that the error message mentions something like “GUID Mismatch” also.
When AxCrypt tries to open or decrypt a file, the first thing it does is check the first 16 bytes of the file content for a “magic sequence of values”, sometimes called a GUID. Each and every file that is encrypted with AxCrypt starts with the same 16 bytes.
This check fails for two known reasons:
1) The file is damaged. This kind of damage is known to occur when the file is encrypted with AxCrypt 1.x, and resides on removable media such as a USB drive, and that media is removed from the computer too quickly without using the “Safe Removal” feature of Windows. In this case, the file may not be completely written, and the first part of the file is lost. This does not typically happen with AxCrypt 2, because we’ve improved the structure of the file in this case to avoid this risk. That’s one of the major reasons AxCrypt 2-files are not quite identical to AxCrypt 1-files.
2) The file is in fact not an AxCrypt file. This typically happens when a user manually renames some file to end with .axx . This causes the file to be associated with AxCrypt, but when AxCrypt tries to open it once again the first thing it does is check those first 16 bytes. If the file then is not an AxCrypt file, you get that message.