If you think it could be a theme or plugin, you could try reproducing the error in safe mode and see if that has any effect on the modal width?
And here are the desktop/mobile view links just in case you’ve styled them out of your forum’s hamburger menu: (though that would make them difficult for a user to accidentally use, but I have seen it happen 
)
Desktop View: [YourSite]/?mobile_view=0
Mobile View: [YourSite]/?mobile_view=1