Dear Pari,
I got the same problem once. What I did that I removed all the registry values related to DI API. Wherever I got the DI API or SAP Business One DI API, I deleted that value from registry. After that, removed client from the system. Cleared Temp and deleted SAP Folder. Restart the system. And make a fresh installation of SAP Client. And it worked for me.
Hope it helps,
Thanks & Regards
Ankit Chauhan