Why does SHA1.ComputeHash fail under high load wit

I'm seeing an issue with some code I maintain. The code below has a private static SHA1 member (which is an IDisposable but since it's static, it should never get finalized). However, under stress this code throws an exception that suggests it has been closed:

Caught exception.  Safe handle has been closed" 
Stack trace: Call stack where exception was thrown
at System.Runtime.InteropServices.SafeHandle.DangerousAddRef(Boolean& success)
at System.Security.Cryptography.Utils.HashData(SafeHashHandle hHash, Byte[] data, Int32 cbData, Int32 ibStart, Int32 cbSize)
at System.Security.Cryptography.Utils.HashData(SafeHashHandle hHash, Byte[] data, Int32 ibStart, Int32 cbSize)
at System.Security.Cryptography.HashAlgorithm.ComputeHash(Byte[] buffer)

The code in question is:

internal class TokenCache
{
    private static SHA1 _sha1 = SHA1.Create();

    private string ComputeHash(string password)
    {
        byte[] passwordBytes = UTF8Encoding.UTF8.GetBytes(password);
        return UTF8Encoding.UTF8.GetString(_sha1.ComputeHash(passwordBytes));
    }

My question is obviously what could cause this issue. Can the call to SHA1.Create fail silently (how many cryptographic resources are available)? Could this be caused by the appdomain going down?

Any other theories?

As per the documentation for the HashAlgorithm base class

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

You should not share these classes between threads where different threads try and call ComputeHash on the same instance at the same time.

EDIT This is what is causing your error. The stress test below yields a variety of errors due to multiple threads calling ComputeHash on the same hash algorithm instance. Your error is one of them.

Specifically, I have seen the following errors with this stress test:

System.Security.Cryptography.CryptographicException: Hash not valid for use in specified state.
System.ObjectDisposedException: Safe handle has been closed

Stress test code sample:

const int threadCount = 2;
var sha1 = SHA1.Create();
var b = new Barrier(threadCount);
Action start = () => {
                    b.SignalAndWait();
                    for (int i = 0; i < 10000; i++)
                    {
                        var pwd = Guid.NewGuid().ToString();
                        var bytes = Encoding.UTF8.GetBytes(pwd);
                        sha1.ComputeHash(bytes);
                    }
                };
var threads = Enumerable.Range(0, threadCount)
                        .Select(_ => new ThreadStart(start))
                        .Select(x => new Thread(x))
                        .ToList();
foreach (var t in threads) t.Start();
foreach (var t in threads) t.Join();