Hey Andrea
I dont think the 2 issues you stated here are related , however could you provide additional details on what changes you did ?
Also please try checking the below :
Attribute BSA defined in the org structur
Correct config under SPRO : Define Backend System for Product Category.
Also ensure that your transaction types and number ranges for PO are appropriate in SRM & ECC.
Also , can you confirm if you use classic / ext classic ?
Please check the below and confirm ?
Check the link below .