OK, I can get this error too.
Rewriting the "save snapshot" function is on my todo list since some
time, because there's something wrong in the current logic for which I
already had to add some hacks (basically, the "save" method is called
while processing the current VBL interrupt, instead of postponing it
after the current instruction is completely processed).
This requires some rather low level changes (interaction with cpu core)
to change this, hopefully I will have more time later to work on this,
but at the moment I'm really busy with too many things to work as much
as I would like on Hatari :(