Where does Yosemite store cache files for exchange accounts? My problem is, that the server of my exchange account crashed today and it is maybe not possible to restore the database of the server.

Are there any files on my Mac or in my TimeMachine BackUp, where for example my notes, or calendar entries are stored for offline use?

I only found the stored data from my iCloud account in the folder:
user / Library / Containers / com.apple.Notes / Data / Library / CoreData / ExternalRecords / B059E164-CC3A-44ED-AB92-C703BDD09455 / IMAPNote / _records /...
You are pretty close for the Notes. If you have an Exchange account set up for Notes then you should have a folder at this location with your Notes.

/Users/USERNAME/Library/Containers/com.apple.Notes/Data/Library/CoreData/ExternalRecords/B059E164-CC3A-44ED-AB92-C703BDD09455/EWSNote/_records/