The version 4 release of neo4j apparently has a breaking change in Cypher, so that expressions of the form {x} now cause an error and must be replaced with $x. This broke the statement template used by the subgraph_exists function (in py2neo.internal.operations.py).
This, in turn, causes the Graph.exists method to ungracefully fail. It appears that similar issues exist with pull_subgraph and push_subgraph.
While the fix is easy and obvious, a patch to py2neo is probably in order.
Thanks,
Tom S.