It wasn’t grayed out yesterday, but it is today. I have no idea how it makes the decision to make adding a source unavailable.
Any ideas?
I just found the same problem in master branch, although I don’t remember for sure when it was last enabled. It may take a bit to dig into the source to figure out why.
This code (at least in current master) is part of alkimia. The method AlkOnlineQuotesWidget::Private::updateButtonState() controls the availability of the New button. Maybe the original author (he is not on this forum from what I know) can provide more details why it is not available under certain circumstances.
I finally went to .config/kmymoney/kmymoneyrc and added a section to put it there.
I’ve downloaded the code to kmymoney previously but it’s really really hard to follow and find the section you need.
Well, the KMyMoney codebase has a history of 20+ years 
Which is no excuse: some parts are really not that easy to understand.