I tried to update to D12 Athens yesterday but that failed for various reasons. But as part of the update I installed the latest Diagram Studio. When I tried to use the existing D11.3 Alexandria I found that that no longer worked. I now get a compile error that DiagramPreview is the wrong version:
([dcc64 Fatal Error] atDiagram.pas(7624): F2051 Unit DiagramPreview was compiled with a different version of atDiagram.TatDiagram).
Comparing the .. /TMS Diagram Studio folder with the one in my still working laptop, I see that the source/design folder, diagramversion.inc and diagram.inc files are missing and the Packages folder is virtually empty (just dclTMSDiagram280.bpl & TMSDiagram280.bpl in d11/Win32/debug and d11/Win64/debug). I did uninstall and re-install Diagram Studio but that made no difference. I am using the latest installer downloaded today.
Update: after a good night's sleep to calm down, I did some searching and found the problem. The old Diagram Studio was in \My Documents\tmssoftware but the new installation is now in C:\Users[me]\AppData\Local\tmssoftware\registered\TMS Diagram Studio. However the Library paths were not updated by the new install.
It's hard to make that claim give the fact your system is messes up with Diagram in different directories. We are not aware of any issues of our installers not updating the library path.
In any case, as usual, the suggested approach is to just uninstall and clean up everything, make sure there are no leftovers, no files left, no wrong Delphi configurations pointing to old components or folders, and only then reinstall. It should work.