Errors and lost my scene!!!!!!!!!!!

Hi,
I was starting up my project and when it was loaded i got the following errors:

Assertion failed on expression: 'iterator == end'

Rebuiling GUID cache: Deleting metadata library/metadata/d6/d676d848669fa1b42a3d93daf3234434 because it is an invalid meta data file!
MetaData class can't be loaded

Unknown error occurred while loading 'Assets/InteractTest.unity'.

Can someone please explain me what i did wrong or what is going on here?!
Is this a Unity bug or a known issue?
All help is Appreciated!!!

 

No, i don't use source control or changed version!

 

I use version 5.5
I thought that there was something wrong with collab or it might be caused by it so turned it off
and surprisingly i could save again. it was like something in collab stopped me of saving and did not reconize the files after a restart of unity.
After turning collab off saving worked again.

However the scene was not really important or something but i tough it was really strange that this happend.
Also a new prefab i created in that lost scene was just blank and could not be used anymore.

 

Sure, I will do that thnx for replying anyway