Hi MartyNZ,
Microsoft broke the O365 sandbox for many sites last Thursday. They're actively working on a fix. If your form uses code (ala qRules), this may explain why it doesn't work in the browser.
If it doesn't use code, then I'm not sure why this isn't working. O365 has changed a lot and it has been a very very rough ride with bugs and breaks left and right. Things stabilized in May and it was smooth sailing for two months until last week. Not sure what to do here, but Microsoft did fix all of the O365 issues from November's abominable upgrade so one would expect this update gets fixed too (if that's the cause of your problem).
Are you using UDCX files for your browser forms?