The data folder.
However, I'm not sure what would be your next step after re-syncing. If you restore from backup, all data will be overwritten.
Which folder i have to delete, i have no data folder or a folder with this name...
As I said before:
The data folder name should be like AppData\Roaming\obyte (not sure, writing from memory, I don't have an easily accessible Windows machine now).
The programme has an option at startup like any other wallet to resynch or rebuild etc.?
No, I see no point in such options for Obyte wallet.