The trick is to use the right namespace, see the xmlns section below.
You should also be sure to re-grant yourself full access with every ACL update.
So your ACL update document should look something like this:
<AccessControlPolicy
xmlns="
http://s3.amazonaws.com/doc/2006-03-01/"> <Owner>
<ID>1234567890</ID>
<DisplayName>pguillard</DisplayName>
</Owner>
<AccessControlList>
<Grant>
<Grantee xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:type="Group">
<URI>
http://acs.amazonaws.com/groups/global/AllUsers</URI>
</Grantee>
<Permission>READ</Permission>
</Grant>
<Grant>
<Grantee xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
<ID>1234567890</ID>
<DisplayName>pguillard</DisplayName>
</Grantee>
<Permission>FULL_CONTROL</Permission>
</Grant>
</AccessControlList>
</AccessControlPolicy>