Calculator has stopped 'converting'...

I haven't looked recently, but "Calculator.app" used to get its exchange rate information by contacting a web page at Apple that in turn got its information by scraping an IMF web page. In the past, if the layout of the original IMF page changed, the Apple page would parse it incorrectly, and "Calculator.app" would retrieve junk, causing currency conversions to fail.

The current version of "Calculator.app" appears to fail to perform any conversions whatsoever (not just limited to currency) if it fails to read the "~/Library/Application Support/Calculator/FinancialRates.xml" file, something it will try to do if there is a currency conversion is in the "Recent Conversions" list.

So until Apple fixes things from their end, if you need to perform currency conversions using "Calculator.app", it will likely be necessary to retrieve a valid "FinancialRates.xml" file from a backup, update it with current information by hand-editing it, then locking it to prevent "Calculator.app" from making further changes. I.e. it would probably be easier to use something else.

If you need to perform conversions other than currency, try deleting the recent conversions list (the property name is ' RecentConversionsDefaultsKey_1' from the "~/Library/Preferences/com.apple.Calculator.plist" file, then refrain from selecting "Currency…" from the "Convert" menu.

As long as there isn't a currency conversion in the "Recent Conversions" list, it should be possible to perform other conversions...

Edit: It appears Apple is getting its info from Yahoo these days, not from the IMF page.