Hi
I just fixed it for myself
Environment Win 10 64 bit Word 2010
The reason: https://docs.microsoft.com/en-us/office/vba/language/reference/user-interface-help/the-code-in-this-project-must-be-updated-for-use-on-64-bit-systems
How to fix
1. Disable Plant UML ( remove PlantUML_Template_v32.dotm from %appdata%\Microsoft\Word\STARTUp)
2. Open PlantUML_Template_v32.dotm in MS Word
3.Close pop ups with error
4. Open ribbon View
5. Press Macros->View Macros
6. Select Macro_UML_all
7. Click Edit
8. fix all non-compiled parts
from Private Declare Function to Private Declare PtrSafe Function
I know, this fix is not for all environments, but you can do this as a quick fix