iCloud contacts disappear and re-appear for seconds on iMac

On my iMac (late 2013 with the current MacOS 10.13.4.) the application "contacts" (version 11.0) does no longer display the iCloud contacts. To be more precise: the contacts disappear and sometimes re-appear, but only for seconds. I tried to de-activate and than re-activate contacts under "Accounts/iCloud" and also tried to re-boot several times - no use. The contacts are 100% available within the iCloud and on all my other devices, both on iOS and MacOS. It is only on this specific device who looses them. Also newly entered contacts disappear after some seconds. Any ideas?