This has been fixed in CVS already.

Confirmed...
Trying to delete the first anchor  lead to removing all anchors, all 
others went fine.
Horde version is 3.3.8

Well, no idea then. I guess you have to trace the code yourself, to 
see where and why this is happening.

mysql> SELECT * FROM horde_syncml_anchors;
+---------------------------------------------+-----------+------------+---------------------+---------------------+
| syncml_syncpartner                          | syncml_db | syncml_uid 
| syncml_clientanchor | syncml_serveranchor |
+---------------------------------------------+-----------+------------+---------------------+---------------------+
| IMEI:356983010663042                        | notes     | user       
  | 20100207T110608Z    | 1265540642          |
| IMEI:356983010663042                        | calendar  | user       
  | 20100207T110608Z    | 1265540642          |
| fmz-nzjzhnSf8prc48+KHJxHJQ==                | tasks     | user       
  | 1265349374          | 1265349378          |
| fmz-nzjzhnSf8prc48+KHJxHJQ==                | calendar  | user       
  | 1265349374          | 1265349378          |
| IMEI:356983010663042                        | contacts  | user       
  | 20100207T110608Z    | 1265540642          |
| sc-pim-0914b4f1-25c9-4766-aa7b-40ee85748fff | contacts  | user       
  | 20100203T190622Z    | 1265223983          |
| fmz-nzjzhnSf8prc48+KHJxHJQ==                | contacts  | user       
  | 1265349374          | 1265349378          |
+---------------------------------------------+-----------+------------+---------------------+---------------------+

I can't see how this is supposed to happen unless the Device or 
Database strings of that anchor are empty.

When i try to delete any syncml-client-anchor it works fine and just 
this anchor is removed, but when i want to delete the first anchor in 
the list, all anchors are getting removed.