Try upgrading to a newer version of the Microsoft .NET Framework
Might wanna use this pack of all essential runtimes. (if thats really cause of the problem)
All in One Runtimes
Unofficial English page: http://s0ft4pc.com/one-runtimes/
Official Deutsch page: https://www.sereby.org/site/All%20in%20One%20Runtimes
Might help avoid any similar issues, when lacking some of the Runtimes.
No additional runtimes are required to run KeePass with KeePassRPC. Newer versions of Windows* even come with a new enough version of .NET to make additional installations unnecessary but if you do need to install the latest version of .NET manually, I recommend going to the Microsoft website and searching for the latest version there rather than utilising an all-in-one installer such as this. Using these sorts of third-party installers comes with some risk of system instability and in some cases unwanted adware and spyware.
KeePassRPC requires .NET 4 or higher but I would recommend always using the latest version you can since bugs will have been fixed since the initial release of v4.
* Definitely 10, probably 8+ too.