The cast_expr_to_bigint function is not implemented in the MSSQL abstraction layer, and as a result, all numeric values being written to the database which should be a BIGINT are being handled instead as INT. As a result, when the upload directory exceeds 2GB, the update of the upload_dir_size in phpbb_config fails with:

SQLSTATE: 22003 code: 8115 message: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]Arithmetic overflow error converting expression to data type int. SQLSTATE: 01000 code: 3621 message: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]The statement has been terminated. [3621]

UPDATE phpbb_config SET config_value = config_value + 157133 WHERE config_name = 'upload_dir_size'

To fix this problem, the mssql_base.php file needs an implementation of cast_expr_to_bigint:

function cast_expr_to_bigint($expression)
{
     return 'CONVERT(BIGINT, ' . $expression . ')';
}

Including this code fixes the problem.