SupportAssist hand uninstalling, then installation again from this place: 
Download Dell SupportAssist

The release number it is the same as the former (2.2.1.23)

Is currently working.
The fault was not mine.

Instead of getting Dell utilities from MS Store, go to Dell Support to get it from Dell. See the video on this page on how to install and register support assist--

http://www.dell.com/support/contents/us/en/04/article/product-support/self-support-knowledgebase/software-and-downloads/supportassist 

Support Assist is an optional utility. If you don't want to use it, uninstall it.

Dear Mary,
I did not ask for it SupportAssyst update, Microsoft Store forcing on me.
I have a well-functioning SupportAssist application, I have been registered and used for a long time.
I know how to do it remove a application.

Your answer does not solve my problem.
Your link will not work my country.

I just said the latest release of SupportAssist did not work.
Sorry.

Same problem.

Now I've got 2: the one I downloaded from Dell's webpage which works fine (ver 2.2.0.253) and the one that I didn't even requested (apparently it was auto installed by MS Store - ver 3.0.0.0). The one that doesn't works hangs on a cmd prompt that says:

SupportAssistUIGroup
 Initiating App Service Connection With UI
Success
Message Sending to WebSocket
_

Scince the one that wont work is a Windows App, I tried to repair - restore from the Windows Apps and Features menu on Settings, but had no luck.

I then decided to unistall (scince it's a Windows App, I right clicked on the app and selected Uninstall) the one that did not work.

Any comments? Why is this happening?

Same here.

I uninstalled everything, reinstalled;  Same issue.  Dell Support Assist 3.0.0.0 comes up with with loading optimization settings and a CMD prompt with a websocket message.

Nothing ever loads.   You just have to close the windows.  

I'm having the same problem starting 3.0.0.0 on my XPS 9560 with Windows 10 Pro. I don't know how the app was installed, but my 2.2.0.244 version is still there.

Launching the new app opens two windows and hangs. The Dell SupportAssist for PCs app window says "Loading PC optimization tools" and the other window is a command window opened by a SupportAssist 3.0.0.0 app called SupportAssistAppWire.exe. The window shows the following text

SupportAssistUIGroup
 Initiating App Service Connection With UI
Success
Message Sending to WebSocket

It would be great to find a solution that either fixes or removes the buggy new app so I can get back to having just one version of SupportAssist on my PC.

Same here, I wondered why I suddenly had two with the newer one not working. I uninstalled it and all is well. Maybe it came with the Windows Spring Update?

"Maybe it came with the Windows Spring Update?"

No, the Microsoft Store sent in it to every Dell computer on May 22, 2018.
You need to remove the application or wait for the fix.

If you want removing it: 
Start menu / Settings / Applications and services / Dell SupportAssist (05.xx.2018) / Remove

Or wait for the official repair...

I got the same problem on the Dell XPS 13 9360 of my daughter (Windows v 1803). going to the start menu, setup, Apps, both the original version and the new version are listed. then I deleted the version 3.0.0.0 

At this point the Store has no longer downloaded the new version and the old one works fine