I finally found the solution for the problem with "interface not supported" when trying to remote-control Word: I had compiled my program for the 32-bit Windows platform, while Office 365 is a 64-bit version. Going back to a 64-bit version, everything worked again! When previously changing the platform, I could not anticipate that side effect. That nasty problem has taken hours to be resolved.
I first tried to find some library for directly manipulating Word files, but could not find anything which looked reasonable. It would not only have to work theoretically, but be documented such that one can use it without wasting a lot of time. Here, many packages fail.
I also previously tried a "repair" of my Office installation, which did not solve the problem but apparently uninstalled Microsoft Teams.