Sorry for not having been clear. Yes, the first plan was to use a different bucket. Then, based on your suggestion, I thought it might be a good idea to reuse the data that is already there, if technically possible. The point here is that the bucket itself got damaged through partially erasing files, but the files that are left should all still be intact. And at a total size of about half a terabyte, I would have saved a lot of time if the exiting chunks could have been reused.
However, in the meantime I tried to reuse the existing chunks but this failed. I created a new Storage that pointed to the bucket with the leftover chunks and ran a check, but the check said “missing chunks”, and the backups failed as well.
So I created a completely new, empty bucket and a new Storage, and now I am backing up everything from scratch. At least, I will have a new backup in a couple of days without tedious re-configuration. Thanks again tangofan for giving the initial tip!