Unfortunatelly the log file can be obtained using command line only. I.e. you would need to have ssh access and execute command:
cr3 --loglevel=TRACE --logfile=/media/sd/cr3.log /media/sd/path to your file/file.epub
or you can copy your epub file to the root of the SD card, rename it to the test.epub and run
script. The cr3.log file will appear in the root of the SD card