We have been having more issues with this library recently when the new version of Roslyn is not able to compile it because it uses corrupted private key, see dotnet/roslyn#32502. We could not find a valid version of the key anywhere so we have tried few workarounds for the library to compile and work but they ended up causing us even more problems. Considering this library is very old and even Linux packagers are shipping their own newer version I think at this point the best way forward is to stop installing in Mono 6.0 and making Mono 5.20 the last version which bundles nunit 2.4.