I got this error when entering a value into a key field of a custom SAP table. I am still not really sure why this error occurred as the solution I found was not really related to the error text as I just reduced the key size of the table.
The overall key length of my table was greater than 120 characters. I therefore got it done to 115 and re-built the SAP table maintenance. After I had done this everything worked fine and was able to add entries with the “_” character via transaction .