Thank you Kghbln,
Having turned on all the debug tools, I found only one message that appeared to sorta-kinda look suspicious;
[GlobalTitleFail] MessageCache::parse called by ParserHook->validateAndRender/ParserHook->renderFatalError/Message->parse/Message->toString/Message->parseText/MessageCache->parse with no title set.
However, and for what it's worth, the actual language of the error message is found on line 83 @
which is part of the data-values/geo package. I'm not sure, but I think that it is installed during the Maps composer installation. In the meantime, I have added "data-values/geo": "1.1.8" into composer.json. No difference was made with 1.1.8 installed with Maps 4.0.5. The parsing errors still occurs.