Right now you need to report that you got the Runtime Compilation error page. That is what was generated by the web server for you and is all you can tell from outside the system, if just using the web to test. Make sure however that you details the environment tested in (os, browser, version, etc) and the steps to reproduce (URL for host and any parameters) and make sure they still reproduce the error (unless it is intermittent, reason unknown).
Separately you should also seek to improve the situation:
The approach to test will also depend on where you are testing.
When testing locally, i.e. running the server locally and using a browser then you can look at the code, the application log that is being generated and its errors and messages.
When testing on staging, work with the developer to add additional information to that error page so that you have more information to report in the case of error. This will benefit local testing also.
If you are testing production, you should find a way for this information to be placed in logs you can access or sent via an email that you can read or a web report you can access. The existing message about runtime compilation is already both too technical and too informative. bots might love it! I would recommend seeking a more generic apology message in the production environment but with details still captured via log and/or email.