For the record: under Win 7, IE 11, the Avira antivirus browser plugin causes Google’s calendar site, GCal, to freeze up and do other strange things. Using just Avira real-time protection, the main anti-virus program, does not cause the problem, just the browser add-on. The odd thing was that Gmail worked fine, but Gcal locked up, or got caught in a scripting error.
The fix is simple: disable the Avira add-on.
And, before you ask, no I don’t use IE much, but I do use it to access a 2nd gcal calendar that belongs to an organization without signing out of Google on my main browser or commingling the two identities in any way.