Show a sample output of your sogod.log file when that happens because we return a 403 and the modification MUST not happen.

(I removed the Postgres log lines)
first invitation created in the webinterface by me:
127.0.0.1 - - [09/May/2011:14:23:13 GMT] "GET /SOGo/so/adi/Calendar/personal//422F-4DC7DC80-55-70B81F00.ics/editAsAppointment?day=20110509&hm=1500 HTTP/1.1" 200 9509/0 0.611 34144 72% 0
2011-05-09 14:23:17.663 sogod[16943] WARNING(-[NSNull(misc) count]): called NSNull -count (returns 0) !!!
127.0.0.1 - - [09/May/2011:14:23:17 GMT] "GET /SOGo/so/adi/Calendar//editAttendees HTTP/1.1" 200 7783/0 0.024 26287 70% 0
127.0.0.1 - - [09/May/2011:14:23:17 GMT] "POST /SOGo/so/adi/freebusy.ifb/ajaxRead?sday=20110502&eday=20110516 HTTP/1.1" 200 103/0 0.027 2879 96% 0
May 09 14:23:21 sogod [16943]: <<0x09B8E298[SOGoContactGCSFolder]:personal>>D fetched 0 records.
127.0.0.1 - - [09/May/2011:14:23:21 GMT] "POST /SOGo/so/adi/Contacts/allContactSearch?excludeGroups=1&search=meetingroom HTTP/1.1" 200 360/0 0.124 - - 0
127.0.0.1 - - [09/May/2011:14:23:23 GMT] "POST /SOGo/so/meetingroom/freebusy.ifb/ajaxRead?sday=20110502&eday=20110516 HTTP/1.1" 200 60/0 0.013 2879 97% 0
127.0.0.1 - - [09/May/2011:14:23:33 GMT] "POST /SOGo/so/adi/Calendar/personal/422F-4DC7DC80-55-70B81F00.ics/saveAsAppointment HTTP/1.1" 200 21/1233 0.160 - - 0

second invitation of a resource from lightning:
127.0.0.1 - - [09/May/2011:14:25:31 GMT] "POST /SOGo/so/user/Calendar/dateselector?day=20110509 HTTP/1.1" 200 795/0 0.013 11049 92% 0
127.0.0.1 - - [09/May/2011:14:25:31 GMT] "GET /SOGo/so/user/Calendar//personal/newevent?day=20110509 HTTP/1.1" 302 0/0 0.008 - - 0
127.0.0.1 - - [09/May/2011:14:25:31 GMT] "GET /SOGo/so/user/Calendar/personal//422F-4DC7DD00-5B-70B81F00.ics/editAsAppointment?day=20110509 HTTP/1.1" 200 8869/0 0.036 31293 71% 0
127.0.0.1 - - [09/May/2011:14:25:43 GMT] "GET /SOGo/so/user/Calendar//editAttendees HTTP/1.1" 200 7782/0 0.026 26291 70% 0
127.0.0.1 - - [09/May/2011:14:25:43 GMT] "POST /SOGo/so/user/freebusy.ifb/ajaxRead?sday=20110502&eday=20110516 HTTP/1.1" 200 79/0 0.026 2879 97% 0
127.0.0.1 - - [09/May/2011:14:25:51 GMT] "POST /SOGo/so/user/Contacts/allContactSearch?excludeGroups=1&search=meetingroom HTTP/1.1" 200 198/0 0.146 - - 0
127.0.0.1 - - [09/May/2011:14:25:57 GMT] "POST /SOGo/so/meetingroom/freebusy.ifb/ajaxRead?sday=20110502&eday=20110516 HTTP/1.1" 200 63/0 0.015 2879 97% 0
127.0.0.1 - - [09/May/2011:14:26:23 GMT] "POST /SOGo/so/user/Calendar/personal/422F-4DC7DD00-5B-70B81F00.ics/saveAsAppointment HTTP/1.1" 200 21/978 0.223 - - 0
127.0.0.1 - - [09/May/2011:14:26:23 GMT] "POST /SOGo/so/user/Calendar/alarmslist?browserTime=1304944087 HTTP/1.1" 200 2/0 0.014 - - 0

Hope that helps.

I don't understand the "second invitation of a resource from lightning:".

This is using "/SOGo/so/" and NOT "/SOGo/dav/" so it's still from the SOGO's web UI.

sorry, my fault: A fresh event created from the web interface:
127.0.0.1 - - [13/May/2011:17:26:03 GMT] "GET /SOGo/so/hmuster/Calendar//personal/newevent?day=20110513&hm=1800 HTTP/1.1" 302 0/0 0.006 - - 4K
127.0.0.1 - - [13/May/2011:17:26:03 GMT] "GET /SOGo/so/hmuster/Calendar/personal//6B6E-4DCD4D80-1-325F9800.ics/editAsAppointment?day=20110513&hm=1800 HTTP/1.1" 200 8855/0 0.070 31309 71% 48K
2011-05-13 17:26:09.202 sogod[27502] WARNING(-[NSNull(misc) count]): called NSNull -count (returns 0) !!!
127.0.0.1 - - [13/May/2011:17:26:09 GMT] "GET /SOGo/so/hmuster/Calendar//editAttendees HTTP/1.1" 200 7781/0 0.033 26300 70% 0
127.0.0.1 - - [13/May/2011:17:26:09 GMT] "POST /SOGo/so/hmuster/freebusy.ifb/ajaxRead?sday=20110506&eday=20110520 HTTP/1.1" 200 52/0 0.013 2879 98% 0
localhost - - [13/May/2011:17:26:12 GMT] "GET /SOGo/dav/georgr/freebusy.ifb HTTP/1.1" 200 898/0 0.207 - - 0
May 13 17:26:14 sogod [27502]: <<0x09391600[SOGoContactGCSFolder]:personal>>D fetched 0 records.
127.0.0.1 - - [13/May/2011:17:26:14 GMT] "POST /SOGo/so/hmuster/Contacts/allContactSearch?excludeGroups=1&search=meetingroom HTTP/1.1" 200 198/0 0.169 - - 24K
127.0.0.1 - - [13/May/2011:17:26:15 GMT] "POST /SOGo/so/meetingroom/freebusy.ifb/ajaxRead?sday=20110506&eday=20110520 HTTP/1.1" 200 88/0 0.155 2879 96% 0
127.0.0.1 - - [13/May/2011:17:26:22 GMT] "POST /SOGo/so/hmuster/Calendar/personal/6B6E-4DCD4D80-1-325F9800.ics/saveAsAppointment HTTP/1.1" 200 21/970 0.166 - - 44K

now I invite "meetingroom" for another event:
May 13 17:27:46 sogod [27501]: <0x09216108[SOGoAppointmentObject]:aeae04ef-7bb6-4592-
bd1c-f33397df2fef.ics> TODO: implement if-none-match for etag: '*'
127.0.0.1 - - [13/May/2011:17:27:46 GMT] "PUT /SOGo/dav/adi/Calendar/personal/aeae04ef-7bb6-4592-bd1c-f33397df2fef.ics HTTP/1.1" 201 0/957 0.165 - - 16K

the event lightning uploaded looks like an ordinary event with:
ORGANIZER;[SNIP];PARTSTAT=ACCEPTED;[SNAP]
ATTENDEE;RSVP=TRUE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT:mailto:meetingroom@...

whereas the other event contains:
ATTENDEE;PARTSTAT=ACCEPTED;RSVP=TRUE;ROLE=REQ-PARTICIPANT;

The webinterface shows both entries in the meetingroom's calendar as accepted -- but as inspecting with cadaver revealed, they are not?

Show us your LDAP source for resources and a LDIF entry.

a resource:
dn: uid=demoroom,ou=SOGo,dc=example,dc=com
objectClass: inetOrgPerson
objectClass: organizationalPerson
objectClass: person
objectClass: top
objectClass: CalendarResource
objectClass: calEntry
cn: Demoroom
sn: Demoroom
displayName: Example Demoroom
givenName: Demoroom
Kind: location
mail: demoroom@example.com
Multiplebookings: 1
uid: demoroom

Resources LDAP Source:
<dict>
<key>CNFieldName</key>
<string>cn</string>
<key>IDFieldName</key>
<string>uid</string>
<key>UIDFieldName</key>
<string>uid</string>
<key>baseDN</key>
<string>ou=SOGo,dc=example,dc=com</string>
<key>bindFields</key>
<array>
<string>uid</string>
</array>
<key>canAuthenticate</key>
<string>YES</string>
<key>displayName</key>
<string>Example Resources</string>
<key>encryption</key>
<string>STARTTLS</string>
<key>hostname</key>
<string>ldap.example.com</string>
<key>id</key>
<string>resources</string>
<key>isAddressBook</key>
<string>YES</string>
<key>port</key>
<string>389</string>
<key>scope</key>
<string>ONE</string>
</dict>
hope this helps. Up to now I did not merge the resources into my main ldap user source. This is pending for once everything works fine. :-)

euh...how is it supposed to work when you haven't defined MultipleBookingsFieldName ?

Thanks for pointing me to this. I misread https://inverse.ca/sogo/lists/arc/users/2011-05/msg00096.html

Both fields are well defined in the LDAP schema; I saw no reason to specify those field names when properly using the scheme.

Sorry for wasting your time; I also updated the page in the wiki accordingly: http://wiki.sogo.nu/ResourceConfiguration

This bug can be closed!

You might want to post in the mailing list to clear that up. It created un-needed confusion.