hcdDefaultDomain and different path

Can you check also what timestamp have Hydra_Core_D7.bpl and Hydra_DX_D7.bpl in <Hydra>\Dcu\D7 folder

I have Hydra_Core_D7, Hydra_IDE_D7 only.

I only deploy Hydra_Core_D7 at my clients, so I only compiled that one, but it does not correct the problem, while the changes you have indicated to me are only in this package.

What are your host and plugin paths?

plugin should in placed into subfolder of folder with host, i.e.
c:\path1\path2\host.exe
c:\path1\path2\path3\plugin.dll

it won’t work, for another case, like:
c:\path1\path2\host.exe
c:\path1\path3\plugin.dll

My application : %PROGRAMFILES%\Name1\Name2\Application.exe
My Plugin : %PROGRAMFILES%\Name3\Name4\Plugin.dll

So, if I do not understand, it can not work?

no, this won’t work.

try to put plugin to
%PROGRAMFILES%\Name1\Name2\Name3\Plugin.dll and retest

Unfortunately, I can not deliver these products as well.

check Remarks section at https://docs.microsoft.com/en-us/dotnet/api/system.appdomainsetup.privatebinpath?view=netframework-4.8 :

so it’s a limitation of .NET.

In your case, ApplicationBase is set to %PROGRAMFILES%\Name1\Name2\ (i.e. host folder).
when %PROGRAMFILES%\Name3\Name4\ path (i.e. plugin folder) is added as PrivateBinPath, it is ignored by .NET according to above Remark.

as a workaround - put RemObjects.Hydra.dll into GAC