RFC of "subst_get_uvers" failed during PREPARE Phase RFCCHK_INI

I am performing a Test upgrade from CRM 5.2 to 6.0 in a Sandbox system and getting the below errors during the RFCCHK_INI phase of PREPARE.

Upg Assistant Error:

ERROR: RFC of "subst_get_uvers" failed.

key : RFC_ERROR_SYSTEM_FAILURE

message : Select one of the installed languages

Error from <upgrade_dir>\put\tmp\dev_RFC.trc

T:9148 Error in program 'SAPup': ======> Select one of the installed languages

T:9148 Error in program 'SAPup': <* RfcReceive [1] : returns 3:RFC_SYS_EXCEPTION

As this seems like it would be a simple issue to resolve, I've verified the following...

- DDIC login to Client 000 is successful

- DDIC pwd is entered correctly into PREPARE Phase INITPUT

- instance parameter zcsa/installed_languages set to "NEFDIOS", which includes all installed languages

- using latest SAPUp version from SMP.

Any ideas what might be happening and/or some additional troubleshooting that can be done?

thank you!

John