As already stated, it's generally an environment issue when something works on the command line, but not from cron. In this specific case, I suspect issue is not an environment variable, though, but rather the working directory. You start off by opening your input file with the assumption that it's in the current directory, but you can't make assumptions like that under cron - unless you've tested, how do you know whether cron runs the script from your home directory, from /tmp, or from just about anywhere else on the system? Does your script work from the command line if you run it from a random, arbitrary directory? (Unless you have a copy of default.db in every directory on the system, I expect not.)

For cron shortcuts, note that 1-59/5 will only run the job 11 times per hour (at :05, :10,... :55 - nothing on the hour). 0-59/5 will get in the 12th run at :00, but if that's what you want, you can do it even more simply with */5, to run whenever the value is a multiple of 5.