This error is easily reproduced, with MSDTC turned on:
BEGIN DISTRIBUTED TRANSACTION;
...
SAVE TRANSACTION abc;
According to this link, the ability to use SAVEPOINTs in a Distributed transaction was dropped in SQL 7, i.e. not a breaking change in SQL 2008 (R2).
It is likely that you previously were NOT running under a Distributed Transaction, but now are, e.g. previously, the remote proc may have been on a different database on the same server, hence no need for a DTC transaction.
So unfortunately, it seems you need to either drop the bounding ACID transaction, or change the SPROC on the remote server so it doesn't use SAVEPOINTs.
Update As per the comments below, it appears that the REMOTE_PROC_TRANSACTIONS option had been set on the OP's newer SQL 2008 server, which automatically begins a distributed transaction across remote stored proc calls.