I use crashplan running on my Nexentastor community edition NAS to back up my files to the cloud. Some time ago, Crashplan stopped officially supporting OpenSolaris as an OS, but the latest version 3.7.0 continued to run fine. I would connect to the headless service using the crashplan app installed on a vm running windows xp, as per the official crashplan documentation. For a while, all was well.
But today I noticed that the crashplan install on my xp vm had a pending update, from 4.2.x to 4.3. It just wasn’t installing, because the gui was never connected to the local crashplan service. I reconfigured the gui (changing the ui.properties file) to connect to the local service, and the install went smoothly. After confirming that the local service was happily running 4.3.0, I went back to the ui.properties file and changed the ServicePort argument to port 4200, which I was port forwarding via putty to my NAS, reopened the gui, and connected to… my local instance.
Wait a minute, that’s not what I was expecting. I should have been connecting to my nas, not the locally running crashplan service. I double checked the ui.properties file but it was fine. Turns out that Crashplan 4.3.0 introduces some additional authentication, and the old 3.7 version of crashplan happily running on my nas could not understand this new format. I searched high and low for information on how to fix this. There’s plenty of information on connecting to a headless 4.x crashplan service, but there seemed to be no information on connecting a 4.3.x gui to a 3.7.x service.
Finally I decided to reinstall crashplan 4.2 on my xp vm. I figured that it would be easy enough to find on the crashplan website, but it turns out it only lists backlevel versions for their enterprise apps. In desperation, I copied the URL of the current version of client, and tried changing the executable name from 4.3 to 4.2. To my surprise, it worked!
The TL;DR version is this: Download CrashPlan_4.2.0_Win.exe then install and run as usual.