Xcode internal errors when testing with swift

I started having peculiar problems when using XCode. Whenever I try to write test with swift, XCode shows an internal error and all the code turns white.

This happens even if I remove the @testable annotation from the code.

I tried reinstalling XCode, but this did not help. The code builds just fine so I don't have a clue where the problem might lie.

I'm unable to properly develop tests this way, my modules are not recognised, I have no error checking or code completion.

Here is a screenshot of how the problem looks:

Would love to get any guidance on this issue...