Bjorn,if you have no backup directory or this directory is empty I doubt there is anything that can be done to restore your project (if you do have backups, please advise). If project.iwprj2 is not zero length, you may have other "paired" xml files, one which has bu in the name (eg fonts.xml & fontsbu.xml, metadata.xml & metadatabu.xml). You could try renaming/moving the filename without the bu and then renaming the matching file by removing the bu from the name.

If this does not work I would assume the project needs to be recreated, if you do this then it is grood practice to occasionally hit the down arrow next to save and select create a backup copy. It is best practice during each session to also select export the project (step 5). This export has never failed to my knowledge (but on rare occasions backups have).

As far as I know WX5 is stable, but if you are using beta software it is essential to take additional measures.