PSADT v4.1.5 | Show-ADTInstallationWelcome | Exception calling "Dispose" with "0" argument(s): "Pipe is broken."

No worries, let's see how it goes. I'm still thinking it's Ivanti but tomorrow will tell. This will actually most definitely be Ivanti as our client/server process communicates via anonymous pipes that only work between the server and client. With Ivanti re-launching the process, it's going to sever that parental relationship and cause the break.

Let's see how you go next time you test, but I'd be excluding anything in the PSAppDeployToolkit folder from Ivanti. Even if it did work, you'd only want the client/server process to be running within the user's privileges anyway.

Can you adjust your rules to exclude elevation of PSADT.ClientServer.Client*.exe?

Wild Success. I put in 2 'Restrict' Elevation rules for:
PSADT.ClientServer.Client.exe
PSADT.ClientServer.Client.Launcher.exe
and that did the trick.

Ivanti Application Manager Event log:
'DWT-DOMAIN\ThurJ' has had user privileges 'modified' for '%systemroot%\ccmcache\31\psappdeploytoolkit\lib\psadt.clientserver.client.exe [Hash:e5d78cf284ce2ae2191622f6e442312a2066c037 Size:233560 bytes] [ProductVersion: 4.1.6.0] [FileVersion: 4.1.6.0] [Product Name: PSAppDeployToolkit Client/Server Client Executable] [Company Name: PSAppDeployToolkit Team] [Vendor: Patch My PC, LLC] [File Description: PSAppDeployToolkit Client/Server Client Executable] [Parent Process: "%systemroot%\ccmcache\31\psappdeploytoolkit\lib\psadt.clientserver.client.launcher.exe" /showmodaldialog -dialogtype customdialog -dialogstyle fluent -options 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]' with policy 'Restrict' on 'PF5GNRCY' in 'Session ID: 2'. Deciding Rule: 'Everyone'. Audit Only mode enabled.

Thanks so much to you both (rockstars) for pointing us in the right direction and helping in getting us over this hurdle.