Mac OS X El Capitan calendar refresh issue

Hello everybody,
maybe we discovered a Mac calendar refresh issue. The appointments don't appear in the native Mac calendar anyway if I choose the 'push' option in the refresh clandar option or 'every minute' instead. Even a manual refresh 'cmd R' won't change anything.

The attaced screenshot shows the GW-webaccess calendar in the background and the the native Mac calendar with the account options in the front (refresh rates every 5/15/30 minutes or every minute/hour or just push)

What might be wrong here?

Thanks and best regards
--
jgoy