Fatal error UnwrappingExceptions during heavy threading

One of our applications just crashed while trying to send an error to Raygun with the following stack trace:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
   at Mindscape.Raygun4Net.RaygunClientBase.StripWrapperExceptions(Exception exception) in e:\\Users\\Jason\\Documents\\GitHub\\raygun4net\\Mindscape.Raygun4Net45\\RaygunClientBase.cs:line 304
   at Mindscape.Raygun4Net.RaygunClientBase.BuildMessage(Exception exception, IList`1 tags, IDictionary userCustomData) in e:\\Users\\Jason\\Documents\\GitHub\\raygun4net\\Mindscape.Raygun4Net45\\RaygunClientBase.cs:line 287
   at Mindscape.Raygun4Net.RaygunClient.<>c__DisplayClassf.<SendInBackground>b__c(Object c) in e:\\Users\\Jason\\Documents\\GitHub\\raygun4net\\Mindscape.Raygun4Net45\\RaygunClient.cs:line 40
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

This particular application consumes multiple messages from a message queue and in this case was handling a big batch of hundreds of messages which all generated errors. It seems like b/c the service uses multiple threads to handle multiple messages concurrently that the large number of errors across a large number of threads exposed a threading issue in the SendInBackground functionality of the Raygun Client.

Thanks Jason, I did a little bit of digging and it appears that the issue stems from the fact that the list of WrapperExceptions is static, but it is also new-ed up every time the constructor is called, which means that if an instance of the client is iterating through the WrapperExceptions list at the same time another instance of the client is being created in another thread, the first instance will be iterating through the collection when the second replaces it with a new one, leading to the error.

We've been able to get around this issue by creating a single client and using it across threads, but this leads to some confusion about how the client is meant to work in a multi-threaded environment. The fact that the WrapperExceptions collection is static seems to indicate that the client is mean to be used as a singleton, but the fact that the affected UserInfo is set on the client would suggest that one is meant to create a new client for every error. Needless to say, this dichotomy has resulted in some serious issues with using Raygun in a multi-threaded environment, and we would appreciate some guidance on how the Raygun team envisioned it being used.