Thank you for your post.
Is the crash occurring at the beginning of the sync? Does the sync complete? If only a partial sync, how many records are updated, and how many remain? This may point to a particular record or group of records.
Have the client reboot FileMaker Go and try again. If this fails, try rebooting the iOS device and try again.
Have you spoken to 360Works about possible causes?
I actually work at 360Works, so we have definitely been contacted haha. We haven't seen this sort of thing before. The crash is happening while locating records by primary key in a table that has many containers in it (mostly 1.5 MB images, with the complete database coming in at ~4 GB). The sync fails once it loses connection with the crashed device, without any records being updated (since this before the actual syncing has started). I've emailed our client to get a copy of his database to reproduce the issue, but haven't heard back yet.
If connection has been lost with the hosted file, I can understand "Lost connection with host" error message, but not a crash. If you receive the file from the customer and are able to reproduce the issue, please let us know. Even better, if you or one of the other members at 360Works can narrow it down further to a group of steps, that may also help us narrow in on the possible cause. Either way, keep me updated.