User received this error (on a build based on 1.0 with GTM-related patches applied).
could not access status of transaction 321456789
DETAIL: Could not open file "pg_clog/0123": No such file or directory.
The clog is the commit log. It is trying to look up a past transaction to see if it committed, but an entire clog's worth of commits is not there.
The current transaction is probably involving a node that was not involved in some previous transactions such that the clog page has not yet been created. XC should probably just create a 0 filled clog page in this case, or assume the XID in question is "uncommitted" on the local node.
Early in XC development some years ago we had some clog issues because not all nodes are involved in all transactions. It appears there still is a clog case to handle.