I have a user on a freshly installed system. The user comes from LDAP. The user is not preconfigured, so there no timezone set (U shows "UTC").

In this case, the events created in the calendar are malformed: the TZID is not set, but a semicolon is still set in the DTSTART entry:

BEGIN:VCALENDAR
PRODID:-//Inverse inc./SOGo 3.2.5//EN
VERSION:2.0
BEGIN:VEVENT
UID:2A-587CC880-7-50573600
SUMMARY:SOGo test entry Mistigkeiten
CLASS:PUBLIC
X-SOGO-SEND-APPOINTMENT-NOTIFICATIONS:NO
TRANSP:OPAQUE
DTSTART;:20170117T170000Z
DTEND;:20170117T180000Z
CREATED:20170116T132040Z
DTSTAMP:20170116T132040Z
LAST-MODIFIED:20170116T132156Z
SEQUENCE:1
END:VEVENT
END:VCALENDAR

This breaks for example the Davdroid implementation:

2017-01-16 14:23:00 1583 [HttpClient$1] <-- END HTTP (393-byte body)
2017-01-16 14:23:00 1583 [ical4android.Event] Parsing iCalendar stream
2017-01-16 14:23:00 1583 [syncadapter.CalendarSyncManager] Received invalid iCalendar, ignoring
EXCEPTION at.bitfire.ical4android.InvalidCalendarException: Couldn't parse calendar resource
at at.bitfire.ical4android.Event.fromStream(Event.java:133)
at at.bitfire.ical4android.Event.fromStream(Event.java:204)
at at.bitfire.davdroid.syncadapter.CalendarSyncManager.processVEvent(CalendarSyncManager.java:209)
...

But it also breaks for example the ics validator at http://severinghaus.org/projects/icv/

Sorry, your calendar could not be parsed.
Error: Error was: Error at line 10:Expected [-3], read [58]
Cause: Sorry, I couldn't get an exact cause.
Context for line 10:
7: CLASS:PUBLIC
8: X-SOGO-SEND-APPOINTMENT-NOTIFICATIONS:NO
9: TRANSP:OPAQUE
10: DTSTART;:20170117T170000Z
11: DTEND;:20170117T180000Z
12: CREATED:20170116T132040Z
13: DTSTAMP:20170116T132040Z