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I have yet to figure out why, but sometimes the MSBuild’s version randomly changes(probably due to updates?). Which cause warnings and errors such as
MSB4019: project loading warning
- Project [project name] load finished with warnings
- Project 'UE5' load finished with warnings
MSBuild version problem
- MSBuild cannot load project [project name]
- MSBuild cannot load project UE5
To resolve this issue in Rider, go to
Settings -> Build, Execution, Deployment -> Toolset and Build -> Toolset

Then, change the MSBuild version to the correct installation. (In my case, the one inside Visual Studio 2022 was the correct one, as installed in THIS link.)
Rider will auto-refresh and resolve the issue.