array_flip error on mail retrive

I get an error on every mail retrive. Cases get generated, but no mail is sent by CT mail.

warning: array_flip() [function.array-flip]: The argument should be an array in E:\program files\Apache Software Foundation\Apache2.2\htdocs\helpdesk\modules\filter\filter.module on line 1306.

Mailhandler retrieve successful: 1 messages for xxx@xxx.xxx

Tested on clean install with only 3 contributed modules.
casetracker 5.x-1.x-dev (Case Tracker, CT Basic, CT mail)
mailhandler 5.x-1.2
smtp 5.x-1.x-dev

server:
WAMP - Apache/2.2.4 (Win32) PHP/5.2.4

mailhandler configuration:
project_number: 100
type: casetracker_basic_case
assign_to: admin
case_status: open
case_priority: 1-high
case_type: bug