switching format .mscz to format musicxml

Hi,
I'm no longer able to switch the format .mscz to format .musicxlm to send to fellow musicians that are using the sybelius software. It was working fine and I don't know what I did. My laptop has a UBUNTU Linux OS.

Up to now I have simply use "export" for a musescore sheet of music and save it in another file using the option .musicxml inthe roll up menu in the bottom right hand corner...and changing the name of the file by substituting musicxml instead of mscz. The process seemed so simple and it was working a few months ago. What am I doing wrong?

The person who received the music file and tries to open and play it on the sybellius software gets and error message, saying that "the file is NOT a musicxml file" and it won't open of course.

Should I just send it in format .mscz and see if the Sybellius software will make the necessary changes and open it in format musicxml?
Thanks
Sally