welcome to the notepad++ community, @Benoit-Martha
your computer “b” most probably has notepad++ 7.6. or higher installed, while computers “a” and “c” have notepad++ 7.5.9 or below.
unfortunately autodetect character encoding is broken since notepad++ version 7.6. and above.
currently it is necessary to:
go to settings > preferences > misc and disable auto detect character encoding on your computer “b”, as seen at the screenshot below.settings - auto detect character encoding
then go to settings > preferences > new document and set your preferred default encoding to e.g utf-8, as seen at the screenshot below.Imgur
after that repeat your testing, it will work on computer “b” now.
note: a solving regression rollback fix for that issue has been committed and pushed to the pull requests, but it has currently been suspended by @donho as the issue is being inspected, to evaluate if a fix is possible, instead of a rollback.