BadImageFormatException is something we have seen recently and our Devs are currently working on the fix. The fix will be coming very soon.
The root cause is a race condition when making IL transformations, which introduces nulls in the transformed IL.
This issue we're seeing is caused by a race condition where the agent injects multiple methods into the application for tracking business transactions. One of the methods injected is calling another method where the method signature exists but the method body doesn't yet. This results in the BadImageFormatException you're seeing.
Meanwhile, we have a workaround configuration to work around this issue without losing visibility. This workaround requires the following changes:
So our recommendation is to make the configuration change above, which will resolve the issue you're seeing now without having to wait for a new agent version. Once the new agent version comes out we recommend upgrading to that version(4.5.18) and then removing the config above from the configuration file.
The .Net agent was updated in December (DOTNET-3700) to fix this issue https://docs.appdynamics.com/display/PRO45/Resolved+Issues+by+Month#ResolvedIssuesbyMonth-January202....
Ryan, AppDynamics Community Manager