That’s the latest version of KeePass and KeePassRPC so it should be working fine (as it does for me on all my machines).
Looking for search results around “ResX file root element missing” seems to bring up issues with .NET or compiling Visual Studio solutions. If you’ve already taken LuckyRat’s advice to update .NET, then I would make a guess that your version of the plugin is unofficial. Unless you specifically chose to rename it to KeePassRPC1.8.0.plgx, the file that’s hosted on GitHub and linked to in the Upgrading KeePassRPC docs should actually be called KeePassRPC.plgx. To be absolutely sure, please check the hashes of your PLGX file and see if they match the following…
MD5: 9D0FAA707EDE47E66681A62B49373468
SHA-1: 66520BC12D54E6090913C0EA64FFAC1DAADB71FC
The file size should be 274,061 bytes. If yours doesn’t match, try downloading the official one again.
If that’s all okay, then it might be a .NET configuration issue and you might need to uninstall all versions of .NET Framework and any associated configuration files stored in your user profile (appdata\roaming etc.) before reinstalling it.
Good luck! 