I run actually kronolith version 4.2.9 and now it seems that the bug 
was already fixed in one of the last versions.

Thanks for your effort.

Still cannot reproduce.

I 've just now testet this with the Evolution CalDAV client.
It's the same behavior as with Thunderbird/Lightning.

Are you sure the base event has really an explicit timezone set in database?
My timezone is Europe/Berlin.

How I can always reproduce this:
- create a recurring event e.g. daily from monday to sunday at 12pm 
with expizit timezone set (with CalDAV client)
- check in database or web GUI if timezone is set really set
- create an exception event e.g. on wednesday to 4pm with expizit 
timezone set (also with CalDAV client)
- now, this exception is lost in my case

If your exception isn't lost in your calendar please check again in 
database or web GUI if the timezone is really set to all events.

While testing with the Evolution client i noticed that it works fine, 
if the base event hasn't set a expizit timezone but the exception 
event has one.
If the the timezone of the base event is set after creating an 
exception event it works also.
But if the exception event is then altered again it is lost.

And another strange behavior occurs on the CalDAV client. If i create 
the base and exception event without timezone it works. But if i 
change the exception event a second time (e.g. to another time) it is 
also lost although there is no timezone set.

I hope i could help you to reproduce this issue.

Cannot reproduce, works fine with and without explicitly set event timezones.

If an exception (also with same or other timezone) of a recurring 
event is created on a CalDAV client, this event isn't created on the 
horde/server side!
This exception event is lost at all.

Existing events and exceptions with timezone are synced fine to CalDAV client.
Also new events and exceptions created in web gui are synced fine.

I think it is a similar behavior as in bug #13369

I use Thunderbird/Lightning as CalDAV client.