Depending on the problem, either nothing or press the "resync" button. Crowdrender checks the blend file on your client/master computer and on the computer/s you want to render on. If there's a difference, then the addon will first attempt to repair the difference. If it can't then the computer will display "sync fail" next to it. 

Pressing the "resync" button will save a copy of the project you have open and upload it to the computer. 

In some cases the difference isn't actually due to your scene, but the differences of Blender's preferences. For example, if you have a computer that has an Nvidia GPU card, it will have an option to use Optix for denoising. If another computer (either your master/client or the computer you want to render on) has not got a similar GPU that can support Optix denoising, then this option doesn't exist.

This missing option is why you may see a persistent problem that can't be fixed by pressing the "resync" button. Don't worry though, in this case, you can actually still render just fine, the "sync fail" warning is just that, a warning. It prompts you to check or do a test render at a lower resolution first to make sure that the scene is going to turn out how you want. 

Connections are required between your client/master computer (where you have Blender and your project open).

To establish a secure connection, a few things are required, which you can usually test;

1. Blender is running and the addon is enabled. We've learned from our own experience, sometimes, something simple like the addon is not running get's overlooked. Make sure by checking in Blender's preferences on all your computers that the addon is enabled!
2. The client/master and the computer you are connecting to are on the same network (this can also be VPN if you want to remotely connect to a computer at your office from your home, or vice versa). The test for this is to use the ping command. If you know the computer's host name/computer name, you can open a shell/terminal or command prompt and type "ping ". You should see replies from the computer, if not, check the connections.
3. Firewalls, by default your firewall, if it's enabled, won't let a program listen for incoming connections, these will be blocked. To test if this is affecting you, temporarily disable your firewall, or, configure a firewall rule to allow the version of Blender you are using to listen on any TCP port. Note that when you install a new version of Blender, most likely you'll need to do this again.
4. Disk space/permissions, Crowdrender makes a copy of your blend file and uploads it to each computer you want to use. This requires a folder to be created to contain the blend file for your project. If the permissions of your user folder (the default location for this folder) don't allow for Blender to create a new folder, then the connection cannot proceed. Also, if there's not enough disk space to upload the blend file, the connection may establish, but the computer won't be able to finish the synchronisation process.

Crowdrender works by automating the following tasks;

1. Establishing a secure connection from your main computer (where you open the blend file and get ready to render) and other computers on the same network as you. 
2. Synchronising your blend file with the other computers you connect to.
3. Coordinating rendering parts of your project on each computer, either by dividing up individual frames (the default for single frame rendering) or dividing up an animation so that each computer renders different frames.
4. Getting the results from each computer as they are rendered, and loading them back into blender as a render result so that Blender can either finish the render, or advance to the next frame in your animation.

Usually the first thing you will do is to add a computer you want to render with. You already have 

We've tested Crowdrender with K-cycles and can confirm that our addons are compatible. Whether you will get a speed up depends!

Basically you're a lot more likely to get a speed up with rendering an animation as each computer will render an entire frame. If you're using another addon to also speed up your rendering then the two will usually combine well.

For single frame renders, if the render time is already small, say less than a minute, then you may or may not see much benefit. 

Also the type and number of computers makes a big difference. If you have a lot of computers, you usually see a bigger speed up. 

If you have computers that are all the same hardware, then you'll get a very consistent speedup. If you have a laptop with a puny CPU rendering with a desktop that has a powerful GPU, then you may find the benefits to be a lot smaller and rendering just on the desktop would be the way to go. 

Crowdrender lets you disable your main computer you're working from and push the render job to another computer, so if you do have a render farm and work from a laptop or workstation, this works really well.

If you enable the addon and get an error, chances are that the software bundled with Blender (python is what we're mainly using in our addon) is not compatible with the version we use in our addon. Refer to the list of Blender versions that is supported for each version of our addon and make sure you are using one that supports your chosen version of Blender.

If the two aren't compatible, consider either downgrading Blender versions or upgrading the addon.