To reproduce:

1. Download attached project "IssuePathTooLong.zip" and open in Unity
2. Create one more subfolder in "ReallyLongFolderNameWoopWoop 5" folder and put in it "behaviourparent.cs" script
Note: You should see in Console these errors:

"DirectoryNotFoundException: Could not find a part of the path "C:\Users\Jonas\Desktop\Projects\960173\Assets\ReallyLongFolderNameWoopWoop\ReallyLongFolderNameWoopWoop 1\ReallyLongFolderNameWoopWoop 2\ReallyLongFolderNameWoopWoop 3\ReallyLongFolderNameWoopWoop 4\ReallyLongFolderNameWoopWoop 5\reallylongfoldernamewoopwoop 6\behaviourparent.cs".
System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/FileStream.cs:292)
System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share)
(wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare)
System.IO.File.OpenRead (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/File.cs:363)
System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding, Boolean detectEncodingFromByteOrderMarks, Int32 bufferSize) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/StreamReader.cs:167)
System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding)
(wrapper remoting-invoke-with-check) System.IO.StreamReader:.ctor (string,System.Text.Encoding)
ICSharpCode.NRefactory.ParserFactory.CreateParser (System.String fileName, System.Text.Encoding encoding)
ICSharpCode.NRefactory.ParserFactory.CreateParser (System.String fileName)
UnityEditor.Scripting.Compilers.CSharpLanguage.GetNamespace (System.String fileName, System.String definedSymbols) (at C:/buildslave/unity/build/Editor/Mono/Scripting/Compilers/CSharpLanguage.cs:44)
UnityEditor.Scripting.ScriptCompilers.GetNamespace (System.String file, System.String definedSymbols) (at C:/buildslave/unity/build/Editor/Mono/Scripting/ScriptCompilers.cs:98)
UnityEditorInternal.InternalEditorUtility:ProjectWindowDrag(HierarchyProperty, Boolean)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
"

"error CS2001: Source file `Assets/ReallyLongFolderNameWoopWoop/ReallyLongFolderNameWoopWoop 1/ReallyLongFolderNameWoopWoop 2/ReallyLongFolderNameWoopWoop 3/ReallyLongFolderNameWoopWoop 4/ReallyLongFolderNameWoopWoop 5/ReallyLongFolderNameWoopWoop 6/behaviourparent.cs' could not be found

Compilation failed: 1 error(s), 0 warnings"

Reproduced on Unity 5.5.5p1, 5.6.4p2, 2017.1.2p2, 2017.2.0p1, 2017.3.0b8 and 2018.1.0a3
On Unity 2017.1.2p2 it shows only "DirectoryNotFoundException: Could not find a part of the path" error, and on Unity 2017.2.0a2 it shows both errors