Many apologies to find you in this trouble.
However, would you mind sharing your .muse file with us so that we can investigate and try fixing it?
For sharing the file you can follow the steps explained here - https://forums.adobe.com/docs/DOC-8652 and send the link to me in a private message.
For doing a private message hover the mouse on my picture and click "message".
It has something to do with the navigation bar. If I delete it then it will export fine, but as soon as I add in any type of navigation I get the same error and Muse crashes. Any ideas?
It is a bug in current version of Muse which can be resolved by a simple workaround.
Please refer to this thread - Original property must have a px value.
On your master page reach out to the small breakpoints where there is an accordion set as the menu.
Right click and select "unlock". Select the accordion and you will find that it is set as "stretch to browser width".
Please change it to "responsive" and things should work fine.
Let me know if you still face any issues.