Okay, it happened again, and this time I at least managed to find an error message. Not sure how useful an error message it is though or if it's even related, I don't remember seeing this any of the previous times:
SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data
e https://test.myth-weavers.com/sheets/index.js:2
postLogin https://test.myth-weavers.com/sheets/index.js:2
AuthProvider https://test.myth-weavers.com/sheets/index.js:2
AuthProvider https://test.myth-weavers.com/sheets/index.js:2
And it...didn't copy the exact point in the code for some reason even though that was in the message and I told it to copy the whole thing. That was 'index.js:2:1148033'