igraph's internal data representation changes sometimes between versions. This means that it is not possible to use igraph objects that were created (and possibly saved to a file) with an older igraph version.
Details
graph_version() queries the current data format,
or the data format of a possibly older igraph graph.
upgrade_graph() can convert an older data format
to the current one.
See also
graph_version to check the current data format version or the version of a graph.
Other versions:
graph_version()