For certain countries (so far only occurred for Singapore) FedEx returns a currency code that is no longer being used (e.g. by XE.com). This causes a currency conversion to fail on e.g. the Shopify side when trying to look up the conversion rate for "SID" currency (supposed to be "Solomon Island Dollar", but FedEx lists it as Singapore Dollars).
Work around the "mistake" on the FedEx part by overwriting SID with SGP currency code. Also double check if any other currency codes might not be correct, although I assume it's a one off here.
FedEX currency code API docs