Hi Diego,
You can check with the following steps for your problem :
a) Check : Have you maintained this FM entry in table SMOFSUBTAB.
b) Check : SMW01 for the BDOC errors. Check log in the transaction. See the data sent by FM .
c) Check : ST22 for dump analysis.
d) Check : Last but not the least check for RFC user access.
Thanks,
Pradeep Singh Dhadwal