Raise verbosity
Make sure you use the -v4 option for extended logging and find logs in the work/images directories.
Getting core files
If CRIU crashes it's useful to build it with debug info and look into core file.
Docker C/R
Docker keeps images in ... and logs in ...
strace
CRIU uses ptrace, so strace can't be used in some cases. perf trace is an alternative way. It isn't so informative, but it is still better than nothing.
Images
The crit tool can help to decode image files into a human readable format.