Internal compiler Error(Unhandled Exception) + all scripts suddenly cant be loaded

Hiya everyone!
I was working through a series of tutorials on making an online FPS using Photon Network. I finished the tutorials and everything worked - players could connect, play together, etc. I launched my project the next day, played it to check if everything worked and then the second time I launched it everything suddenly broke.
I get the following error:

Internal compiler error. See the console log for more information. output was:
Unhandled Exception: System.TypeLoadException: Could not load type ‘System.Func`1’ from assembly ‘mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e’.

at (wrapper managed-to-native) System.Reflection.MonoMethodInfo:get_method_info (intptr,System.Reflection.MonoMethodInfo&)

at System.Reflection.MonoMethodInfo.GetMethodInfo (IntPtr handle) [0x00000] in :0

at System.Reflection.MonoMethodInfo.GetAttributes (IntPtr handle) [0x00000] in :0

at System.Reflection.MonoMethod.get_Attributes () [0x00000] in :0

at System.Reflection.MethodBase.get_IsVirtual () [0x00000] in :0

at Mono.CSharp.MemberCache.AddMethods (BindingFlags bf, System.Type type) [0x00000] in :0

at Mono.CSharp.MemberCache.AddMethods (System.Type type) [0x00000] in :0

at Mono.CSharp.MemberCache…ctor (IMemberContainer container) [0x00000] in :0

at Mono.CSharp.TypeHandle…ctor (System.Type type) [0x00000] in :0

at Mono.CSharp.TypeHandle.GetTypeHandle (System.Type t) [0x00000] in :0

at Mono.CSharp.TypeHandle.GetMemberCache (System.Type t) [0x00000] in :0

at Mono.CSharp.TypeManager.MemberLookup_FindMembers (System.Type t, MemberTypes mt, BindingFlags bf, System.String name, System.Boolean& used_cache) [0x00000] in :0

at Mono.CSharp.TypeManager.RealMemberLookup (System.Type invocation_type, System.Type qualifier_type, System.Type queried_type, MemberTypes mt, BindingFlags original_bf, System.String name, IList almost_match) [0x00000] in :0

at Mono.CSharp.TypeManager.MemberLookup (System.Type invocation_type, System.Type qualifier_type, System.Type queried_type, MemberTypes mt, BindingFlags original_bf, System.String name, IList almost_match) [0x00000] in :0

at Mono.CSharp.Expression.MemberLookup (Mono.CSharp.CompilerContext ctx, System.Type container_type, System.Type qualifier_type, System.Type queried_type, System.String name, MemberTypes mt, BindingFlags bf, Location loc) [0x00000] in :0

at Mono.CSharp.Expression.MemberLookup (Mono.CSharp.CompilerContext ctx, System.Type container_type, System.Type qualifier_type, System.Type queried_type, System.String name, Location loc) [0x00000] in :0

at Mono.CSharp.MemberAccess.DoResolve (Mono.CSharp.ResolveContext ec, Mono.CSharp.Expression right_side) [0x00000] in :0

at Mono.CSharp.MemberAccess.DoResolve (Mono.CSharp.ResolveContext ec) [0x00000] in :0

at Mono.CSharp.Expression.Resolve (Mono.CSharp.ResolveContext ec, ResolveFlags flags) [0x00000] in :0

at Mono.CSharp.Invocation.DoResolve (Mono.CSharp.ResolveContext ec) [0x00000] in :0

at Mono.CSharp.Expression.Resolve (Mono.CSharp.ResolveContext ec, ResolveFlags flags) [0x00000] in :0

at Mono.CSharp.Expression.Resolve (Mono.CSharp.ResolveContext ec) [0x00000] in :0

at Mono.CSharp.Return.DoResolve (Mono.CSharp.BlockContext ec) [0x00000] in :0

at Mono.CSharp.ExitStatement.Resolve (Mono.CSharp.BlockContext ec) [0x00000] in :0

at Mono.CSharp.Block.Resolve (Mono.CSharp.BlockContext ec) [0x00000] in :0

at Mono.CSharp.ToplevelBlock.Resolve (Mono.CSharp.FlowBranching parent, Mono.CSharp.BlockContext rc, Mono.CSharp.ParametersCompiled ip, IMethodData md) [0x00000] in :0

It won’t let me build or play my game anymore.
Also all of the scripts I created show up as “can’t be loaded” in Unity. I literally made no changes between the two starts of Unity and am baffled at what could have broken. Apparently it’s an error with .NET (from my googling) and the .dll files within the plugin folder but they worked perfectly yesterday…
Any help would be largely appreciated!

Guessing you are working on the Multiplayer FPS tutorial from Quil, if so you need to move the plugin folder from Photon (the one with Metro and WP8 subfolders) into the root (directly in the Assets folder).

(it is explained in video 4 of the tutorial).