Most probably an SQL-statement overflow i reckon.
When you have a lot of IN-parameters you can get problems.
The SAP System creates a SQL statement with OR from it, you dont need to care about that.
Problem is, that the field which holds the SQL-statement that is beeing sent to the DB has a limited length. As far as i know it is 255Kb. So when you got LOTS of IN-parameters the created statement exeeds 255Kb and thus leads to a system failjure.
Try with less IN-Parameters or if not applicable, use "FOR ALL ENTRIES"-statement, tho i guess that cant be done using RFC_READ_TABLE.