Apple shifts in OS X 10.9 (Mavericks) from AFP file sharing to SMB2. However, the current implementation seems to be a buggy and is not compatible with my NAS system from Synology.
A workaround is to force the using of SMB1. This could be done by the creation of the file ~/Library/Preferences/nsmb.conf with following content:
If Apple solves this problem, you could switch back to SMB2 by deleting the file. With
you will get further information about the SMB configuration file.
Edit: It seems to be a bug in Synology DiskStation Manager (DSM) und its fixed in their new release.