Hi Petr,
For a given <customer_number>, please check if there is a corresponding entry in table CRMM_BUT_CUSTNO. Enter <customer_number> from the SLG1 error message into the CUSTOMER_NO field of table CRMM_BUT_CUSTNO and let us know what you find.
When you create a BP in CRM, a BDoc goes to ECC to create the corresponding Customer, and then a feedback from ECC comes to CRM informing CRM which is the corresponding customer number for a particular BP GUID. This is stored in CRMM_BUT_CUSTNO. I suppose in your case, there is no record for this customer in CRM in this table ( ECC Customer -> CRM BP mapping is missing ).
In this case, it will be necessary to fix the mapping in CRM/ECC and then download the condition records again.
Best regards,
Vijayalakshmi