Calendar Server error 504 and 'Server with secure communication unavailable'

After upgrade to Yosemite 10.10.4 and server 4.1.3 (from Mavericks and 3.2.2) Calendars became unavailable. I don't seem to get any farther than this error message:

7/26/15 3:15:56.794 PM Calendar[46384]: [com.apple.calendar.iaplugin.log.CalInternetAccountsPlugin] [Autodiscovery error: [Error Domain=CoreDAVHTTPStatusErrorDomain Code=405 "The operation couldn’t be completed. (CoreDAVHTTPStatusErrorDomain error 405.)" UserInfo=0x600000263f80 {CoreDAVHTTPHeaders=<CFBasicHash 0x600000471a80 [0x7fff79c97ed0]>{type = immutable dict, count = 7,

entries =>

0 : Server = Apache

1 : Content-Type = <CFString 0x60800064c240 [0x7fff79c97ed0]>{contents = "text/html; charset=iso-8859-1"}

2 : Allow = <CFString 0x608000450d40 [0x7fff79c97ed0]>{contents = "POST,OPTIONS,GET,HEAD"}

6 : Date = <CFString 0x6080006409f0 [0x7fff79c97ed0]>{contents = "Sun, 26 Jul 2015 19:15:56 GMT"}

10 : Keep-Alive = <CFString 0x608000645490 [0x7fff79c97ed0]>{contents = "timeout=15, max=100"}

11 : Content-Length = 308

12 : Connection = <CFString 0x608000436be0 [0x7fff79c97ed0]>{contents = "Keep-Alive"}

}

}].]

caldavd is not shown as a process in Activity Monitor, but I get these messages on the server.

7/26/15 3:25:44.843 PM com.apple.xpc.launchd[1]: (org.calendarserver.calendarserver) Service only ran for 34 seconds. Pushing respawn out by 26 seconds.

I get no indication that anything to do with Calendar on the Server has been contacted. Open Directory, Kerberos, and User Home Folders working fine. Can ssh in fine.

This produces the same symptoms even if I work only on the Server itself and use the Server Admin user to create a calendar for a local network User (possibly bypassing network configuration issues).

I am perfectly willing to trash all pre-existing Calendars (not using Contacts) and start over. any insights appreciated.