Error after upgrading write plugin code to 15.1

We have recompiled our Write plug-in code with 15.1 dll's and .NET 4.8, after deployment we are running into a security error as follows: Has anyone else seen this?

The plug-in "xxxxxxxxx" returned the following error: "Trisoft.InfoShare.Core.Identity.Exceptions.IdentityException: Security error.

 at Trisoft.InfoShare.Core.RepositoryContext.Create(ClaimsPrincipal claimsPrincipal, IRepositoryFormatSettings formatSettings)

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