Thanks for your fast reply, Garima.
I tried the described steps using Firefox with no success. Afterwards I changed the default browser application to Chrome again AND deleted the browsers cache.
I could successfully sync now establishing the connection via Chrome.
Could you please check with a developer why old cookies (I assume that's the reason) could cause sync issues? I don't think the browsers cache should be important when establishing a new connection to iCloud as Enpass already states that a re-authentication is necessary to sync.