This only occures, if you don't have modify privileges for this event/calendar.
IMHO this is correct behaviour.

No, this is not the case - it happens even when the user is the owner of the entry's calendar.

My user has his "personal" calendar and one additional calendar "hannes". When I edit an item in "hannes", the dropdown defaults to "personal".
(In fact, the dropdown always defaults to "personal").

This is a snippet from the popup's HTML:
<select name="2.1.0.3.0.1.5.3.1.3.1.5.2.4" id="calendarList">
<option value="0">Personal Calendar</option>
<option value="1">cal 1</option>
<option value="2">Hannes</option>
<option value="3">cal 4</option>
<option value="4">cal 4</option></select>

As you can see, there is no option pre-SELECTED, and I think that is the issue.

What have you set in "Preferences" --> "Calendar Options" --> "Default Calendar:"

It should be "selected calendar".

The setting already is "selected calendar". I just tried changing it, but that does not make a difference: when editing, there's always the first calendar selected in the dropdown, not the actual one.

Sorry, but I don't get this behaviour, neither in 2.0.7 nor in 2.1.1a.
I get a selected option in the popup:
<option value="1" selected="selected">Calendar for testing</option>

Which browser are you using?

I tried various browsers with no differences:

Firefox 25.0.1, Chromium 30.0.1599.114 (on Ubuntu 13.10)
IE 8 on Windows XP

It appears I haven't found a way to see any entry of the calendar dropdown "selected".
However the other dropdowns, category and priority, work fine.

I also tried the very same browsers at the Inverse SOGO demo installation: there, the correct calendar is selected.

What I did notice is on my install, the data javascript doesn't have the "activeCalendar" value set. That is:

MY INSTALL:
var activeCalendar = '';
var activeComponent = '4071-528D2180-1DB-7796D700.ics';

sogo-demo.inverse.ca:
var activeCalendar = '6E71-52989400-1-5C319680';
var activeComponent = '6E71-52989400-5-5C319680.ics';

Thank you very much for your support!

Did you update your sope packages as well?

Yes, SOGo 2.1.1a is compiled against SOPE 2.1.1; both are installed and according to lsof | grep sogo the correct files are used.

Does your webserver by chance deliver some old static content like *.js files?

I cleaned /usr/local/lib/GNUstep/SOGo before "make install"ing the new SOGO version and there is no caching configured on the Apache side.

Do you think the empty javascript variable might be a clue or is this just coincidence?

We are experiencing this same issue. We see no <option> marked selected, and we also see "var activeCalendar = '';" in the source for the pop-up.

We are using version 2.2.13-1 installed from the .deb. Here is some output for dpkg show the versions of sope libraries too.

dpkg -l | grep 'sope\|sogo' returns:
"""
ii libsope-appserver4.9 4.9.r1664.20141230 amd64 SOPE application server libraries
ii libsope-core4.9 4.9.r1664.20141230 amd64 Core libraries of the SOPE application server
ii libsope-gdl1-4.9 4.9.r1664.20141230 amd64 GNUstep database libraries for SOPE
ii libsope-ldap4.9 4.9.r1664.20141230 amd64 SOPE libraries for LDAP access
ii libsope-mime4.9 4.9.r1664.20141230 amd64 SOPE libraries for MIME processing
ii libsope-xml4.9 4.9.r1664.20141230 amd64 SOPE libraries for XML processing
ii sogo:amd64 2.2.13-1 amd64 a modern and scalable groupware
ii sope4.9-gdl1-postgresql 4.9.r1664.20141230 amd64 PostgreSQL connector for SOPE's fork of the GNUstep database environment
ii sope4.9-libxmlsaxdriver 4.9.r1664.20141230 amd64 XML Parser for SOPE's SAX engine
"""

Please let me know if you'd prefer me to raise this as a separate bug against version 2.2.13

This regression is fixed in the upcoming v2.2.14.