I have just downloaded the latest version of the owncloud client for Mac OS X (2.5.3, build 11290), and when starting it get a critical issue in Little Snitch because there is apparently a problem with code signature of the application. Here is the Little Snitch full report:
priority: extra high
notes: On 2019-02-19, owncloud tried to establish a connection to xxxxxxx. The request was denied automatically during Silent Mode because the code signature does not match what existing rules require.
This error never occurred with the previous version of the owncloud client.
Client version: 2.5.3, build 11290
Operating system: Mac OS X 10.14.3
OS language: English