I have found the solution. It comes from:
I have nextcloud 11.0.3
I have applied all hints but helps only:
So, in lib/base.php find:
$incompatibleUserAgents = [
// OS X Finder
Now I can synchronize events between server and mobile.
But not all works properly.
I have removed in browser the calendar (all) and then I have done synchronization. And the error has occurred (404). The calendar which does not exist on the server can not be remove from mobile.