It seems my following query is failing and throwing "Cannot complete this action".However when I test this query in CAML Query builder it's working fine.
<Where>
<And>
<Or>
<Eq><FieldRef Name='Participant' /><Value Type='User'>Test1</Value></Eq>
<Eq><FieldRef Name='Participant' /><Value Type='User'>Test2</Value></Eq>
<Eq><FieldRef Name='Participant' /><Value Type='User'>Test3</Value></Eq>
</Or>
<Eq><FieldRef Name='Department' /><Value Type='Text'>Positioning</Value></Eq>
</And>
</Where>