It appears that CrashPlan 3.5.3 bundles a buggy version of the "Eclipse Standard Widget Toolkit". To fix, you just need to replace that toolkit with the latest one from the developer.
* Download swt-4.3-gtk-linux-x86.zip and swt-4.3-gtk-linux-x86_64.zip from [http://download.eclipse.org/eclipse/downloads/drops4/R-4.3-201306052000/#SWT Eclipse].
* Extract swt.jar from the x86_64 file and rename it as swt-64.jar, and extract swt.jar from the second file. I saved them both to the home directory. (You can do this from terminal from the directory in which you saved the .zip files):
unzip -e swt-4.3-gtk-linux-x86_64.zip swt.jar -d ~
mv ~/swt.jar ~/swt-64.jar
Then you need to copy these to the CrashPlan library. On my HDA CrashPlan was installed to /var/hda/web-apps/crashplan/html. If your installation is different you will need to replace the commands below this your directory.
* Stop the crashplan server
sudo systemctl stop crashplan.service
* Back up the old files and copy in the new ones (change the directory path to your own if it is different and remember to put the /lib at the end):
cd /var/hda/web-apps/crashplan/html/lib
mv swt.jar swt.jar.old
mv ~/swt-64.jar swt-64.jar
* Restart the crashplan
systemctl start crashplan.service