I had a strange issue where CrashPlan Pro wouldn't start up. I kept getting the error "Cannot connect to backup engine". After reviewing some <a href="http://support.code42.com/CrashPlan/Latest/Troubleshooting/Unable_To_Connect_To_The_Backup_Engine" target="_blank" rel="noopener noreferrer">help docs from CrashPlan</a> it was still not working. Every time I tried to start the CrashPlanService it would start and immediately stop. The log files weren't much help to me, they were just letting me know what I already knew, it had started and stopped for some strange reason.
A lot of the results that come up when you Google "CrashPlan Pro cannot connect to backup engine" show solutions where users were able to increase their memory from 512M to 1024M and it would resolve it. For me, when I went to C:\Program Files\CrashPlan\CrashPlanService.ini and checked what was there, it already had 1024M, so I figured maybe it wanted more. So I set it to 2048M to see if that would work. Nope.
After fruitlessly trying other things such as updating Java, turning it off and then on again, saying pretty please, I finally caved and sent their support team a message about it along with the log files from my attempts to start the service. They replied back within a few hours that I should try lowering the memory used from 1024M to 512M. So I gave that a shot, and to my surprise it started up perfectly! My guess is that since the computer has some other memory intensive applications running, it was trying to use more memory than was available at the time.
Solution
So the final solution was to open up the file C:\Program Files\CrashPlan\CrashPlanService.ini and to update -Xmx1024m to -Xmx512m