Hi there! This may indicate a real problem with your card templates - 2.15 began parsing card templates more strictly.
However, you don't specify your exact version. There was an issue in 2.15.0 and 2.15.1 where spaces were not handled great and I fixed that in 2.15.2 just last evening, that could be it. Or it could be that your back template is actually specifying a "Pinyin" field that does not exist on the set of fields for the card and you should use the card template editor to remove the reference to that non-existent field
If you can verify this card template still works on Anki Desktop (also free, if you don't have it yet and you sync to it for free via AnkiWeb) in its current version then please open an issue so we may fix it
https://docs.ankidroid.org/help.html
-Mike