This is commonly caused by the GAB folder not being exported properly. This can be verified using the following steps:

If your logging looks similar to the following, then the GAB folder is not properly exported through Z-Push or the exported folder is empty.

2016/05/30 16:28:18.162 (1,VSTA_Main): Trace: GAB: FullResync begin: 0
2016/05/30 16:28:18.162 (1,VSTA_Main): Debug: ZPushLocalStore: Opening store with prefix C:\Users\user\AppData\Local\Kopano\Kopano OL Extension\KopanoLocalFolders
2016/05/30 16:28:18.162 (1,VSTA_Main): Info: ZPushLocalStore: Opening existing store: C:\Users\user\AppData\Local\Kopano\Kopano OL Extension\KopanoLocalFolders.pst
2016/05/30 16:28:18.162 (1,VSTA_Main): Trace: GAB: FullResync done: 0

 

Further indicators of a not properly exported GAB folder are an empty value at GABFolderLinked in the Debug settings dialogue:

And a missing "GAB" entry in the synchronized data row of "z-push-admin -a list -u user":