A Unicode enabled remote depot can only be used with a Unicode enabled server. If the remote depot is Unicode enabled, the main server must be Unicode enabled, as well.
To enable switching a Perforce Server instance to run in Unicode mode, you must run the p4 -xi command on the Perforce Server database files (in $P4ROOT). More information on enabling Unicode mode can be found in the Perforce Administrator's Guide section on Managing Unicode installations.
Converting a server to Unicode mode is a one-way operation! You cannot restore a Unicode server to its previous state.