KyPass use the official SDK from cloud provider to synchronize your databases.
In the v5.1, the SDK for Dropbox-sdk-obj-c use the v3.11.1. This SDK is buggy… and fixed in v3.11.2 released yesterday.
It means that you couldn’t synchronize anymore with Dropbox on v5.1.
v5.2 was sent to Apple today. Expect it in a few hours.
Sorry for the inconvenience