Hi Carlos,
this error message BBP_PD 176 should be issued only if different Logical Systems of the ECC Backend are mixed in one Purchasing Document. And if this is the case you should not be able to bring the Purchasing Document to Status "Awaiting Approval". Please have a look at this:
http://wiki.scn.sap.com/wiki/display/SRM/PO+items+for+different+Product+Categories
Please check if the Purchasing Document giving the error has multiple Logical Systems referenced in the various LOGSYS fields SRM has. If they are all the same within SRM I suggest you create an incident in SAPs OSS. If they are not all the same you may hove to implement some enhancements (BADIs) Controlling the Logical Systems are consistent.
Regards
Oliver