Hi Charlie,
Would you be able to check this version for me and see if that fixes the problem: https://github.com/mendix/Calendar/raw/master/test/widgets/calendar.mpk
I think it has to do with a logger statement that tries to log the MxObject. I have seen this issue before in new Mendix versions where the MxObject logging resulted in an error somewhere.
I'll wait for your feedback. If it fixes it, I'll release this fixed version as 5.4.1 in the AppStore.