Cannot open *.plt File in linux

[routeconverter]

My symptom in post #2 indicates that a external GPSBabel is adressed in the Windows version although no external GPSBabel is specified in RouteConverter options.

That is true: RouteConverter tries to be clever to reduce the number of support requests.
In BabelFormat#findBabel you find the current resolution:

Code:

private String findBabel() throws IOException {
        // 1. check if there is a preference and try to find its file
        File babelFile = getBabelPathPreference() != null ? new File(getBabelPathPreference()) : null;
        if (babelFile == null || !babelFile.exists()) {
            babelFile = null;
        }

        // 2a. look for "c:\Program Files\GPSBabel\gpsbabel.exe"
        if (babelFile == null && Platform.isWindows()) {
            babelFile = checkIfBabelExists(System.getenv("ProgramFiles") + "\\GPSBabel\\gpsbabel.exe");
        }

        // 2b. look for "c:\Program Files (x86)\GPSBabel\gpsbabel.exe"
        if (babelFile == null && Platform.isWindows()) {
            babelFile = checkIfBabelExists(System.getenv("ProgramFiles(x86)") + "\\GPSBabel\\gpsbabel.exe");
        }

        // 3. look for "/usr/bin/gpsbabel" in path
        if (babelFile == null && !Platform.isWindows()) {
            babelFile = checkIfBabelExists("/usr/bin/gpsbabel");
        }

        // 4. extract from classpath into temp directrory and execute there
        if (babelFile == null) {
            String path = Platform.getOperationSystem() + "/" + Platform.getArchitecture() + "/";
            if (Platform.isWindows()) {
                Externalization.extractFile(path + "libexpat.dll");
                babelFile = Externalization.extractFile(path + "gpsbabel.exe");
            } else if (Platform.isLinux() || Platform.isMac()) {
                babelFile = Externalization.extractFile(path + "gpsbabel");
            }
        }

        // 4. look for unqualified "gpsbabel"
        return babelFile != null ? babelFile.getAbsolutePath() : "gpsbabel";
    }

Is a internal GPSBabel delivered with the linux version as well as with the Windows version?

Yes, but it might be too old such that current Linux distributions do not install the compatibility libraries which it requires by default. It's a 1.3.6 statically compiled with Ubuntu 09.04 (?) or 10.04