Unity 5.4.0f3 Failed assemblies stripper

Hi all,
currently im trying to get Stripping Level: Strip Byte Code to work on our Project, but i just can't get it to work and im kinda lost after searching on multiple posts adressing similar problems.

Tried many different things:

• Setting up a Link.xml for Security.Cryptography, in the end i ditched the Script which uses that namespace (got a different error message after that)
• Tried setting Stripping Level to Strip assemblies (still same error message)
• Tried the newest Unity Version (2017.1) (Didn't work either)
• Deleting all Plugins (still same error message)

Current Settings:

• Api Compatibility Level: .NET 2.0
• Stripping Level: Strip Byte Code

Building with Stripping Level: Disabled works but the file size is not acceptable unfortunatly.

Is there a problem with using System ? If it is it would be quite a hassle to convert a lot of our scripts.

If required i can upload our Editor.log after build.

Error Message:

 

Does someone have any ideas?