Short answer: turning off the calendars for the account and then turning them back on again (and maybe temporarily disabling and re-enabling the account) fixed it.
Details:
Turning off "Calendars" for that account removed the calendars from Calendar.app (not surprisingly). Turning them back on again showed no calendars for about 10 minutes. During that time I opened the Console, turned on live streaming and I see an unending spam of messages at 10Hz+ from exchangesyncd process, mostly:
default 09:21:51.855508-0700 exchangesyncd Connection interrupted
default 09:21:51.855747-0700 exchangesyncd [0x86585f840] Re-initialization successful; calling out to event handler with XPC_ERROR_CONNECTION_INTERRUPTED
but also stuff like:
default 09:22:11.715909-0700 exchangesyncd Change Item property_blob size: 5322
default 09:22:11.717092-0700 exchangesyncd pushing items [(
"<0x83e30e1c0> Added Canceled: DriveUX: Dev-QA sync up <NSDateComponents: 0x883963590> {\n TimeZone: America/Denver (MST) offset -25200\n Era: 1\n Calendar Year: 2019\n Month: 7\n Day: 2\n Hour: 10\n Minute: 0\n Second: 0\n Nanosecond: 0 <NSDateComponents: 0x8839634b0> {\n TimeZone: America/Denver (MST) offset -25200\n Era: 1\n Calendar Year: 2019\n Month: 7\n Day: 2\n Hour: 11\n Minute: 0\n Second: 0\n Nanosecond: 0 Event Item change 13649 internal (null) item 13210 SyncProtocol-EWS-0E0BFC6C-F845-443A-BAE6-B98336F55515"
)]
error 09:22:11.720241-0700 exchangesyncd Error getting event with uniqueId F52A93B6-54F9-40B4-8209-35E209D0EE0E/RID=583776000: Error Domain=EKCADErrorDomain Code=1010 "Object not found. It may have been deleted." UserInfo={NSLocalizedDescription=Object not found. It may have been deleted.}
default 09:22:11.725795-0700 exchangesyncd creating event [<0x83e30e1c0> Added Canceled: DriveUX: Dev-QA sync up <NSDateComponents: 0x883963590> {
TimeZone: America/Denver (MST) offset -25200
Era: 1
Calendar Year: 2019
Month: 7
Day: 2
Hour: 10
Minute: 0
Second: 0
Nanosecond: 0 <NSDateComponents: 0x8839634b0> {
TimeZone: America/Denver (MST) offset -25200
Era: 1
Calendar Year: 2019
Month: 7
Day: 2
Hour: 11
Minute: 0
Second: 0
Nanosecond: 0 Event Item change 13649 internal (null) item 13210 SyncProtocol-EWS-0E0BFC6C-F845-443A-BAE6-B98336F55515]
error 09:22:11.727365-0700 exchangesyncd Error getting event with uniqueId F52A93B6-54F9-40B4-8209-35E209D0EE0E/RID=583776000: Error Domain=EKCADErrorDomain Code=1010 "Object not found. It may have been deleted." UserInfo={NSLocalizedDescription=Object not found. It may have been deleted.}
error 09:22:11.730119-0700 exchangesyncd Tried to set proposed start date on invalid event: <EKEvent rowid=[16952] UUID=[716ADEF7-FE8B-485F-B17E-D44200878936] start-date=[2019-07-02 16:00:00 +0000]>
Eventually the calendar showed up in Calendars.app again, with the errorneous events gone. The exchange sync process has also finally settled down.
