What do you mean by "crash"? As in the device reboots? Turns off?
If it reboots, that's probably the watchdog timer forcing a reboot because RMSDK is taking too long to open the file and thinks something is wrong. This has happened to me with large databases and basically boils down to running low on RAM.
One thing you could try is rebooting the device to free up RAM, then immediately open the book once you're logged in. Else, sometimes it'll open if you wait long enough, but unfortunately, sometimes sickel won't let you. There's a patch that can increase the timeout to 30 seconds, but I don't know if it was updated for firmware 4.26. If that's not long enough, you can also
disable sickel entirely (will need to be reflashed after every firmware update); it may take 2 minutes to open the file, but it might open if you wait long enough. If it takes too long, you can always hard power off.
Otherwise, no idea. I've started using koreader or plato for things in epub format and just use nickel for kepubs.