Yesterday evening users could create quotes from opportunities, but today they cannot anymore. Nothing has changed in our system, I can vouch for that. I checked incoming change sets, audit trail, nothing. Also, there are just two admins, and yesterday evening we haven't done anything.
The symptom is that no opportunity fields are copied to the New Quote screen, so you cannot save the quote.
I have already filed a case at Salesforce Support, I was wondering if other people have the same problem.
Update 1: the above "New Quote" button is the standard Salesforce button. Another user noted that a custom button, that uses a URL hack to fill fields when creating a new Account, also has the problem that fields are no longer filled. So there seems to be a wider problem where values from the URL are no longer copied to fields in the window.
Update 2: as an attempt for a workaround, I created a custom List button at the quote object with contents: /lightning/o/Quote/new
and added it to the quote related list of an opportunity.
This pops up the new quote window, with empty fields of course, but just as in the earlier screenshot, there is no possibility to fill the name of the linked opportunity.
Update 3: this problem only happens in Lightning Experience. As an ultimate workaround, switch to Classic, create the quote there and switch back to Lightning.