When using Client Manager 8.5 to connect to a region upgraded to Gen 8.6, the following messages occur.
CICS Log: F169 20210603090550 TIRCFTO IEFT Error: National Language Translation Failed
Client Manager Log: ERROR - CONNECTION RESET BY PEER
Release : 8.6
Component : Gen Run Time, Distributed
The outgoing buffer from the Client Manager is using codepage 1252. So, the 1252<->037 tables (or whatever code page being used on the mainframe) have not been built into your translation runtime on the mainframe.