Thanks for the post - I feel your frustration. I don’t like to assume but can I assume that you’re using Mockups 3 for Desktop on macOS and you’ve recently updated to 10.15.1 (Catalina)?
If not, please let me know which platform you’re on and which OS version you’re using and we’ll continue to work through this.
If you are on Catalina, it’s a bit of a bad news/good news situation.
The bad news is that this is caused by the Catalina upgrade - it’s an issue with the underlying framework we use (Adobe Air).
The good news is that there is a reasonably painless workaround that will allow you to save/export your work.
If you just choose a different folder (e.g. try the default ~/Downloads or ~/Desktop folders), you’ll be able to save and export without issue. The issue seems to impact networked/virtual drives (e.g. Dropbox or Google Drive) so I’d advise avoiding those when saving directly (you could still save locally then copy project files to those types of drives and vice versa).
It’s frustrating that we haven’t been able to fix this yet but we are at the mercy of Adobe and Harman (who support the Air platform) here. As soon as they release a fix, we will update the app with that fix.
Can you try the simple workaround and let me know if that allows you to save? I’ll be standing by here, Chris.