After saving the XML file, the directory containing the file becomes locked and cannot be deleted.

[softmgr]

For more details about this issue, see:
notepad-plus-plus/notepad-plus-plus#16974

I have checked the XMLTools source code and identified the root cause. The fix needs to be made in ToolsXmlValidate.cpp:

int performXMLCheck(int informIfNoError) {
    dbgln("performXMLCheck()");

    // -1. save old current folder
    wchar_t oldPath[MAX_PATH]{};
    if (!_wgetcwd(oldPath, MAX_PATH)) {
        // Failed to retrieve folder
        return -1;
    }

    // ...
    // ...

    // 1. Restore the original folder
    _wchdir(oldPath);

    return res;
}

void XMLValidation(int informIfNoError) {
    dbgln("XMLValidation()");

    // -1. save old current folder
    wchar_t oldPath[MAX_PATH]{};
    if (!_wgetcwd(oldPath, MAX_PATH)) {
        // Failed to retrieve folder
        return;
    }

    // ...
    // ...

    // 2. Restore the original folder
    _wchdir(oldPath);
}

Until this issue is resolved, you can temporarily disable it by unchecking Enable XML syntax auto-check in the plugins menu.