System.InvalidOperationException when opening project in Unity 2017

When I open my project in Unity3D (2017.1.1f1 64 bit) I'm getting the following error (twice):

System.InvalidOperationException: Operation is not valid due to the current state of the object. at Mono.Cecil.ModuleDefinition.ProcessDebugHeader () [0x00021] in :0
at Mono.Cecil.ModuleDefinition.ReadSymbols (Mono.Cecil.Cil.ISymbolReader reader) [0x00015] in :0
at Mono.Cecil.ModuleReader.ReadSymbols (Mono.Cecil.ModuleDefinition module, Mono.Cecil.ReaderParameters parameters) [0x00047] in :0
at Mono.Cecil.ModuleReader.CreateModuleFrom (Mono.Cecil.PE.Image image, Mono.Cecil.ReaderParameters parameters) [0x0007b] in :0
at Mono.Cecil.ModuleDefinition.ReadModule (System.IO.Stream stream, Mono.Cecil.ReaderParameters parameters) [0x00028] in :0
at Mono.Cecil.AssemblyDefinition.ReadAssembly (System.IO.Stream stream, Mono.Cecil.ReaderParameters parameters) [0x00000] in :0
at AssemblyUpdater.Core.AssemblyUpdaterContext.ReadAssembly (System.String assemblyPath, APIUpdater.Framework.Log.IAPIUpdaterListener listener, System.IO.FileAccess mode, System.String[] searchPaths) [0x0007c] in :0 > at > AssemblyUpdater.Core.AssemblyUpdaterContext.From > (System.String assemblyPath, > APIUpdater.Framework.Configuration.IConfigurationProvider > configuration, System.String[] > assemblySearchPaths, > AssemblyUpdater.Core.UpdaterMode mode, > APIUpdater.Framework.Log.IAPIUpdaterListener > listener) [0x0002b] in > :0 > at > AssemblyUpdater.Core.AssemblyUpdaterContext.From > (System.String assemblyPath, > System.String[] assemblySearchPaths, > AssemblyUpdater.Core.UpdaterMode mode, > APIUpdater.Framework.Log.IAPIUpdaterListener > listener) [0x00001] in > :0 > at > AssemblyUpdater.Application.Program.CheckForObsoleteAPIUsage > (AssemblyUpdater.Application.CommandLineSpec > config) [0x00013] in > :0 > at > AssemblyUpdater.Application.Program.Main > (System.String[] args) [0x00057] in > :0 > UnityEditor.Scripting.APIUpdaterHelper:DoesAssemblyRequireUpgrade(String)

I'm lost because I haven't been able to find any references to this on the internet, and it just started appearing recently (and when I try to go back to the way the project was before the changes, the errors don't disapppear.

I've also fully uninstalled unity3d (including appdata and registry keys), but the error doesn't disappear.

I've enabled experimental 4.6 compatibility, and my dlls are built to target 4.6 (error also appears on 4.5.2)

What could be the cause of this issue?

Thank you for any help.