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 320 and 600 px 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.
Thank you so much !!