Excel Web App error everyday after recreation

Question

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.

Answer 1

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.

Answer 2

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.