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I try to View/Edit Excel documents (xlsx) in Office Web App SP1. I immediately get an error. "An unexpected error has occurred.". Word and Powerpoint both work fine. I have recreated the Excel Service Application with success, but it shows the same exact error and ULS logs the next day...

This is in my ULS Logs:

Process: w3wp.exe
Level: Unexpected

System.Security.SecurityException: That assembly does not allow partially trusted callers. at System.Security.CodeAccessSecurityEngine.ThrowSecurityException(Assembly asm, PermissionSet granted, PermissionSet refused, RuntimeMethodHandle rmh, SecurityAction action, Object demand, IPermission permThatFailed) at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandle& ctor, Boolean& bNeedSecurityCheck) at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean fillCache) at System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache) at System.Activator.CreateInstance(Type type, Boolean nonPublic) at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) at Microsoft.Office.Excel.Server.MossHost.MossHost.TryCreateHostExtension(String className, IHostExtension& extension) at Microsoft.Office.Excel.Server.MossHost.MossHost.LoadHostExtensions()

Just a couple notes:
Excel Services is running.
Office Web Apps feature is enabled on the site collection.
Word and Powerpoint work fine.

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I did a Office Web App repair that fixed it, but i'll see if that sticks tomorrow. – Mike Feb 8 '12 at 22:42
Error reoccurred this morning... – Mike Feb 9 '12 at 14:25

I can offer to you a potential workaround. Maybe it's not best practice but may be acceptable in your environment. You can set trust mode in web application config to Full:

<trust level="Full" originUrl="" />

It hepled me in similar situation (the same "That assembly does not allow partially trusted callers" exception) once. It's turn off some code access security restrictions. At least it may hepls you until you'll find the real problem.

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It's not worth it. It's only Excel web application. Everything else works great. – Mike Feb 10 '12 at 19:39
So you can configure this option in Excel web application, can't you? – Alex Boev Feb 10 '12 at 19:44
I want to stay in good practice anyways. There has to be another way. – Mike Feb 10 '12 at 19:50
Whats the current build level for SharePoint? If you can go to a hotfix it might be worth it. August 2011 CU has been well tested. – pigeon Feb 11 '12 at 6:45
My build is 14.0.6029.1000 on SharePoint Server and Office Web Apps. – Mike Feb 13 '12 at 22:22
up vote 0 down vote accepted

I decided to Uninstall Office Web Apps completely. Beware! It will disconnect you from the farm so keep your farm passphrase handy. I will reinstall Office Web Apps at a later date.

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