I also failed to update/manually enter my weight. No problem to enter the numbers but an error message appears the moment you say 'Okay'.
Here, it will work as long as the weight is an integer (no digit past the decimal point), i.e 72.0kg is accepted whereas 72.1kg isn't ("no available data"). It works fine from Garmin Connect on the web, though (and these updates do replicate to the Garmin Connect Mobile app on the smartphone)...
(Garmin Connect Mobile for Android v2.3.3 on Android 4.4.2)
Since latest android app update, the scroll wheel of weight values becomes transparent when trying to use it. It still works, but you can't see what the numbers are. Version 2.4
EDIT: never mind, seems to be working fine now... Weird, because it behaved like I described above several times on different days.