OCS Presence unknown

We have a remote office connected via VPN and and ASA firewall.
When users login to OCS they are able to login but cannot see presence of other contacts, or serarch for contacts or talk/chat to other users. Strangely if the user login as one user and start OCS as another user typing in the user id and password it works. its same behaviour for different accounts we tested. Link is quiet stable and we are able to telnet to OCS ports 5060,5061. can any one help, error message is given below.

Event Type:   Warning
> Event Source:          Communicator
> Event Category:       None
> Event ID:       11
> Date:            19-09-2011
> Time:            17:30:20
> User:            N/A
> Computer:     Computer35
> Description:
> A SIP request made by Communicator failed in an unexpected manner (status code 80ee000c). More information is contained in the following technical data:
> RequestUri:   sip:user1@domain.com
> From:         sip:user2@domain.com;tag=8429823754
> To:           sip:user1@domain.com;tag=(null)
> Call-ID:      9eb3f3f07abd4ba2ae47ca84f09149b7
> Content-type: application/sdp;call-type=im

> v=0
> o=- 0 0 IN IP4
> s=session
> c=IN IP4
> t=0 0
> m=message 5060 sip null
> a=accept-types:text/plain multipart/alternative image/gif text/rtf
> text/html application/x-ms-ink application/ms-imdn+xml
> text/x-msmsgsinvite

> Response Data:

> 2163081228  (null)
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HemachandranConnect With a Mentor Author Commented:
It was ASA Firewall issue. Removing package inspection in ASA fixed the issue.
no fixup protocol SIP 5060
HemachandranAuthor Commented:
Further update, it looks starnge if I delete most of my added contacts from another site PC then it works. When we have many contacts in OCS it looks like trying to contact all the exchange servers and I can see heavy CPU usage also. Then it stops working while with few say 10 contacts its working fine. But then the issue is I do not get any result while searching in OCS. Search for new users gives no result.
HemachandranAuthor Commented:
No one responded and the issue was identified internally. Submitting to help someone in future.
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