I think it is related to freetds configuration. I recommend you try to connect the target MS SQL Server via the tsql client of FreeTDS. You may need to adjust tds protocol version and character set.
I have created a foreign table on my postgresql server to select the data on external table which is reside on MS SQL Server, everything went well from installing to freetds and installing and configuring tds_fdw tool, and created foreign server and user mapping then created foreign table, but when i do select from table_name then it throws an error which is i mentioned below as.
ERROR: DB-Library error: DB #: 20018, DB Msg: General SQL Server error: Check messages from the SQL Server, OS #: -1, OS Msg: , Level: 16
will be highly grateful for your valuable suggestions, where to look for to get this done.