The RAP-155 and IAP-204 use different firmware 'classes'. The RAP-155 uses 'Aries' and the IAP-204 uses 'Taurus' so depending on how these IAPs are managed there is the possibility for issues when pulling down an IAP image.
I would suggest moving the RAP-155 to a separate VLAN from the IAP-204 , upgrading the RAP-155 to the same firmware version (e.g 6.5.x) as the IAP-204, then re-introduce the RAP-155 to the IAP-204 cluster.