I lost the last attempt at reporting this, sorry if it double posts.
If you try to save a project under another name using file/save as, if the project has not already been saved at all you get the following error in the log
mxWM_SaveWBL error:
- cannot get WBL
<edit>This happens with a straight save as well. It may be because I have not calculated the water table. That's the only difference I can see.</edit>